arm64: mm: add missing P4D definitions and use them consistently

arm64: mm: add missing P4D definitions and use them consistently

Even though level 0, 1 and 2 descriptors share the same attribute
encodings, let's be a bit more consistent about using the right one at
the right level. So add new macros for level 0/P4D definitions, and
clean up some inconsistencies involving these macros.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20210310104942.174584-2-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>