docs: arm64: Add layout and 52-bit info to memory document

As the kernel no longer prints out the memory layout on boot, this patch
adds this information back to the memory document.

Also, as the 52-bit support introduces some subtle changes to the arm64
memory, the rationale behind these changes are also added to the memory
document.

Signed-off-by: Steve Capper <steve.capper@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>