arm64: Introduce {set,clear}_pte_bit

It's useful to be able to change individual bits in ptes at times.
Introduce functions for this and update existing pte_mk* functions
to use these primatives.

Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>
[will: added missing inline keyword for new header functions]
Signed-off-by: Will Deacon <will.deacon@arm.com>