arm64/mm: Add uffd write-protect support

Let's use the newly-free PTE SW bit (58) to add support for uffd-wp.

The standard handlers are implemented for set/test/clear for both pte
and pmd. Additionally we must also track the uffd-wp state as a pte swp
bit, so use a free swap pte bit (3).

Acked-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>