Documentation/arm64: clarify requirements for DTB placement

The current description of DTB placement requirements does not quite
match the kernel code in head.S: __vet_fdt and __create_page_tables.
This patch tweaks the text to match the actual requirements placed on
it by the code.

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>