arm/mmu: Fix boot on beagle-xm

* The changes for pi2 support led to the virtual addresses overlapping
with the page table again on the beagle, because the kernel address
space overlaps with the physical RAM identity mapped. Try to find a
memory range in a way that will work in both cases.

src/system/boot/arch/arm/arch_mmu.cpp

@@ -668,7 +668,8 @@ mmu_init(void)
 			return /*B_ERROR*/;
 #endif
 		}
-		dprintf("total physical memory = %" B_PRId64 "MB\n", total / (1024 * 1024));
+		dprintf("total physical memory = %" B_PRId64 "MB\n",
+			total / (1024 * 1024));
 	}
 
 	// see if subpages are disabled
@@ -685,10 +686,17 @@ mmu_init(void)
 
 	// Mark start for dynamic allocation
 	sNextPhysicalAddress = sPageTableRegionEnd;
+	// We are going to allocate early kernel stuff there, so the virtual address
+	// has to be in kernel space. Either put it right after the identity mapped
+	// RAM (if it happens to be in the 0x80000000 range), or at the start of
+	// the kernel address space otherwise.
 	sNextVirtualAddress = KERNEL_LOAD_BASE + kMaxKernelSize;
+	if (sPageTableRegionEnd > sNextVirtualAddress)
+		sNextVirtualAddress = sPageTableRegionEnd;
 
 	// mark allocated ranges, so they don't get overwritten
-	insert_physical_allocated_range((addr_t)&_start, (addr_t)&_end - (addr_t)&_start);
+	insert_physical_allocated_range((addr_t)&_start,
+		(addr_t)&_end - (addr_t)&_start);
 	insert_physical_allocated_range((addr_t)sPageDirectory, 0x200000);
 
 	init_page_directory();