arm64/mm: Refactor PMD_PRESENT_INVALID and PTE_PROT_NONE bits

Currently the PMD_PRESENT_INVALID and PTE_PROT_NONE functionality
explicitly occupy 2 bits in the PTE when PTE_VALID/PMD_SECT_VALID is
clear. This has 2 significant consequences:

  - PTE_PROT_NONE consumes a precious SW PTE bit that could be used for
    other things.
  - The swap pte layout must reserve those same 2 bits and ensure they
    are both always zero for a swap pte. It would be nice to reclaim at
    least one of those bits.

Note that while PMD_PRESENT_INVALID technically only applies to pmds,
the swap pte layout is common to ptes and pmds so we are currently
effectively reserving that bit at both levels.

Let's replace PMD_PRESENT_INVALID with a more generic
PTE_PRESENT_INVALID bit, which occupies the same position (bit 59) but
applies uniformly to page/block descriptors at any level. This bit is
only interpreted when PTE_VALID is clear. If it is set, then the pte is
still considered present; pte_present() returns true and all the fields
in the pte follow the HW interpretation (e.g. SW can safely call
pte_pfn(), etc). But crucially, the HW treats the pte as invalid and
will fault if it hits.

With this in place, we can remove PTE_PROT_NONE entirely and instead
represent PROT_NONE as a present but invalid pte (PTE_VALID=0,
PTE_INVALID=1) with PTE_USER=0 and PTE_UXN=1. This is a unique
combination that is not used anywhere else.

The net result is a clearer, simpler, more generic encoding scheme that
applies uniformly to all levels. Additionally we free up a PTE SW bit
and a swap pte bit (bit 58 in both cases).

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>