arm64/mm: Wire up PTE_CONT for user mappings

With the ptep API sufficiently refactored, we can now introduce a new
"contpte" API layer, which transparently manages the PTE_CONT bit for
user mappings. Whenever it detects a set of PTEs that meet the
requirements for a contiguous range, the PTEs are re-painted with the
PTE_CONT bit.

This initial change provides a baseline that can be optimized in future
commits. That said, fold/unfold operations (which imply tlb
invalidation) are avoided where possible with a few tricks for
access/dirty bit management.

Write-enable and write-protect modifications are likely non-optimal and
likely incure a regression in fork() performance. This will be addressed
separately.

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