mm: Add new ptep_deref() helper to fully encapsulate pte_t

There are many call sites that directly dereference a pte_t pointer.
This makes it very difficult to properly encapsulate a page table in the
arch code without having to allocate shadow page tables. ptep_deref()
aims to solve this by replacing all direct dereferences with a call to
this function.

The default implementation continues to just dereference the pointer
(*ptep), so generated code should be exactly the same. However, it is
possible for the architecture to override the default with their own
implementation, that can (e.g.) hide certain bits from the core code, or
determine young/dirty status by mixing in state from another source.

While ptep_get() and ptep_get_lockless() already exist, these are
implemented as atomic accesses (e.g. READ_ONCE() in the default case).
So rather than using ptep_get() and risking performance regressions,
introduce an new variant.

Call sites will be converted to use the accessor in future commits.

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