efi/dtb: Find potential FDT on UEFI

* Makes our UEFI bootloader somewhat FDT/DTB aware on all
  architectures.
* Will report when an FDT is found, and provide it to kernels
  that want it.

Change-Id: I90324fc0579a9c835e60568fa9b654c2df0aba27
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3543
Reviewed-by: Fredrik Holmqvist <[email protected]>
Reviewed-by: Adrien Destugues <[email protected]>

headers/private/kernel/arch/arm/arch_kernel_args.h

@@ -30,6 +30,7 @@ typedef struct {
 
 	// needed for UEFI, otherwise kernel acpi support can't find ACPI root
 	FixedWidthPointer<void> acpi_root;
+	FixedWidthPointer<void> fdt;
 } _PACKED arch_kernel_args;
 
 #endif	/* KERNEL_ARCH_ARM_KERNEL_ARGS_H */

headers/private/kernel/platform/efi/types.h

@@ -68,6 +68,9 @@
     {0xeb9d2d31, 0x2d88, 0x11d3, {0x9a, 0x16, 0x00, 0x90, 0x27, 0x3f, 0xc1, 0x4d}}
 #define SMBIOS3_TABLE_GUID \
     {0xf2fd1544, 0x9794, 0x4a2c, {0x99, 0x2e, 0xe5, 0xbb, 0xcf, 0x20, 0xe3, 0x94}}
+#define DEVICE_TREE_GUID \
+    {0xb1b621d5, 0xf19c, 0x41a5, {0x83, 0x0b, 0xd9, 0x15, 0x2c, 0x69, 0xaa, 0xe0}}
+
 
 typedef struct {
     uint64_t Signature;