KVM: selftests: arm64: Determine max ipa size per-page size

We are about to add 52 bit PA guest modes for 4K and 16K pages when the
system supports LPA2. In preparation beef up the logic that parses mmfr0
to also tell us what the maximum supported PA size is for each page
size. Max PA size = 0 implies the page size is not supported at all.

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