KVM: arm64: Support upto 5 levels of translation in kvm_pgtable

FEAT_LPA2 increases the maximum levels of translation from 4 to 5 for
the 4KB page case, when IA is >48 bits. While we can still use 4 levels
for stage2 translation in this case (due to stage2 allowing concatenated
page tables for first level lookup), the same kvm_pgtable library is
used for the hyp stage1 page tables and stage1 does not support
concatenation.

Therefore, modify the library to support upto 5 levels. Previous patches
already laid the groundwork for this by refactoring code to work in
terms of KVM_PGTABLE_FIRST_LEVEL and KVM_PGTABLE_LAST_LEVEL. So we just
need to change these macros.

The hardware sometimes encodes the new level differently from the
others: One such place is when reading the level from the FSC field in
the ESR_EL2 register. We never expect to see the lowest level (-1) here
since the stage 2 page tables always use concatenated tables for first
level lookup and therefore only use 4 levels of lookup. So we get away
with just adding a comment to explain why we are not being careful about
decoding level -1.

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