KVM: arm64: Use LPA2 page-tables for stage2 and hyp stage1

Implement a simple policy whereby if the HW supports FEAT_LPA2 for the
page size we are using, always use LPA2-style page-tables for stage 2
and hyp stage 1, regardless of the VMM-requested IPA size or
HW-implemented PA size. When in use we can now support up to 52-bit IPA
and PA sizes.

We use the previously created cpu feature to track whether LPA2 is
supported for deciding whether to use the LPA2 or classic pte format.

Note that FEAT_LPA2 brings support for bigger block mappings (512GB with
4KB, 64GB with 16KB). We explicitly don't enable these in the library
because stage2_apply_range() works on batch sizes of the largest used
block mapping, and increasing the size of the batch would lead to soft
lockups. See commit 5994bc9e ("KVM: arm64: Limit
stage2_apply_range() batch size to largest block").

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