arm64/mm: set_ptes()/set_pte_at(): New layer to manage contig bit

Create a new layer for the in-table PTE manipulation APIs. For now, The
existing API is prefixed with double underscore to become the
arch-private API and the public API is just a simple wrapper that calls
the private API.

The public API implementation will subsequently be used to transparently
manipulate the contiguous bit where appropriate. But since there are
already some contig-aware users (e.g. hugetlb, kernel mapper), we must
first ensure those users use the private API directly so that the future
contig-bit manipulations in the public API do not interfere with those
existing uses.

set_pte_at() is a core macro that forwards to set_ptes() (with nr=1).
Instead of creating a __set_pte_at() internal macro, convert all arch
users to use set_ptes()/__set_ptes() directly, as appropriate. Callers
in hugetlb may benefit from calling __set_ptes() once for their whole
range rather than managing their own loop. This is left for future
improvement.

No behavioural changes intended.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>