arm64: mmu: architecture specific hooks for demand paging support

A page table entry used for demand paging is always from the last level
page table and never completely zeroed. Anything else is considered
ARCH_DATA_PAGE_NOT_MAPPED.

Loaded pages use a PTE_PAGE_DESC table entry type. Paged-out pages use a
PTE_INVALID_DESC table entry type and the physical address field is
reused to hold the backing store location token value.

ARCH_DATA_PAGE_ACCESSED corresponds to the AF flag. It is initially
unset and manually set to catch when pages are being accessed and
eventually do something about it.

ARCH_DATA_PAGE_DIRTY corresponds to the lack of the AP_RO flag.
Similarly to the AF flag, pages are initially made read-only to catch
writes.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>