add some docs for agp_gart and intel_extreme drivers

Not a lot of info there yet, but it's a start.

docs/develop/busses/agp_gart/ReadMe.md

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+AGP (and PCI-express) Graphics Address Re-Mapping Table
+=======================================================
+
+The GART is an IO-MMU allowing the videocard and CPU to share some memory.
+Either the CPU can access the video RAM directly ("aperture"), or the video
+card can access the system RAM using DMA access.
+
+The GART converts between physical addresses and virtual addresses on the
+video card side. Of course, the CPU must then map these physical addresses
+in its own address space to use them (using the MMU).
+
+The GART works as you'd expect from an MMU. It has a page table (called GTT)
+in RAM and walks it to figure out mappings. Since there cannot be page misses
+(that would require exception handling on the GPU side), access to missing
+pages are instead sent to a dedicated "scratch" page which is not used for
+anything else.
+
+Our driver implements the GART and GTT for Intel graphics card only, so far.
+Since our videodrivers are only doing modesetting, they do not need much
+support and other drivers implemented GTT management directly on their own
+(it is usually enough to make the framebuffer accessible to the CPU). However,
+this could be generalized into a more flexible iommu bus protocol.

docs/develop/drivers/intel_extreme/generations.txt

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+Intel video hardware generations
+================================
+
+This file summarizes the different generations of Intel hardware, because the
+naming is a bit inconsistent and it's hard to follow which is which sometimes.
+The devices can be referred to by year of introduction, generation number,
+chipset commercial number, or internal codenames.
+
+Generation 1
+============
+
+These are the i740 and i810 devices handled by intel_810. No further info will
+be provided here.
+
+Generation 2 / 2002
+============
+
+i830, 845, 85x, 865
+
+Generation 3 / 2004
+============
+
+This is the first generation to be documented at intellinuxgraphics.org.
+Generation 2 devices are quite similar for the modesetting part, but not
+identical.
+
+GMA 900 (i915G)
+GMA 950 (i945G)
+GMA 3000 (946GZ, Q965, Q963)
+GMA 3100 (G31, G33, Q33 et Q35)
+GMA 3150 (Pineview for Atom CPUs)
+
+Generation 4 / 2006
+============
+
+GMA X3000 (i965G)
+GMA X3100 (i965GM)
+GMA X3500 (G35)
+GMA 4500 (Q43, Q45)
+GMA 4500M / 4500HD (GL40, GS45, GM45, GM47)
+GMA X4500 / X4500HD (G41, G43 (X4500), G45 (X4500HD))
+
+Generation 5 / 2010
+============
+
+Westmere / Clarkdale, Arrandale / Iron Lake / Ibex Peak
+
+Switches from the traditional northbridge / southbridge to the new
+"platform control hub" design. Essentially, most of the northbridge functions
+are now directly in the CPU package.
+
+Generation 6 / 2011
+============
+
+Sandybridge / Cougar Point
+
+The northbridge and CPU are now even on the same die.
+
+Generation 7 / 2012
+============
+
+Ivy Bridge / Panther Point and Haswell / Lynx Point
+
+A lot of the video output hardware is moved from the northbridge (CPU) to the
+PCH. This makes sense because it allows to match the PCH chipset (soldered on
+to the motherboard) with the video ports (also soldered there). Otherwise, the
+CPU generation would define which ports are usable or not.
+
+This impacts several things in the modesetting sequence, as well as the address
+of the registers which are moved.
+
+This is also the first generation to support 3 independant displays, which
+also impacts the register layout in many places.
+
+Generation 8 / 2013
+============
+
+Broadwell / Wildcat Point and Braswell
+
+Generation 9 / 2015
+============
+
+Skylake / Sunrise Point, Apollo Lake, Kaby Lake / Union Point
+
+Generation 10
+=============
+
+Cannon Point / Coffee Lake
+
+Generation 11
+=============
+
+Ice Lake
+
+Generation 12
+=============
+
+Tiger Lake