arm64/mm: Move PTE_INVALID to overlay PTE_NS

PTE_INVALID was previously occupying bit 59, which when a PTE is valid
can either be IGNORED, PBHA[0] or AttrIndex[3], depending on the HW
configuration. In practice this is currently not a problem because
PTE_INVALID can only be 1 when PTE_VALID=0 and upstream Linux always
requires the bit set to 0 for a valid pte.

However, if in future Linux wants to use the field (e.g. AttrIndex[3])
then we could end up with confusion when PTE_INVALID comes along and
corrupts the field - we would ideally want to preserve it even for an
invalid (but present) pte.

The other problem with bit 59 is that it prevents the offset field of a
swap entry within a swap pte from growing beyond 51 bits. By moving
PTE_INVALID to a low bit we can lay the swap pte out so that the
offset field could grow to 53 bits in future.

So let's move PTE_INVALID to overlay PTE_NS (bit 5). PTE_NS is res0 for
SW outside of the secure state so Linux will never need to touch it.

These are both marginal benefits, but make things a bit tidier in my
opinion.

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