arch/arm64/mmu: move hardware specifics to private header

None of the moved definitions are meant to be used by any code outside
of arch/arm64/core/mmu.c. Move them away from global scope to the
private header where more such definitions already live.

This is especially relevant as the previous commit fixed some of those
definitions which then caused conflicts with some external SDK that
carries a copy of those original buggy definitions.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>