arm64: Add ARM64_HAS_LPA2 CPU capability

Expose FEAT_LPA2 as a capability so that we can take advantage of
alternatives patching in the hypervisor.

Although FEAT_LPA2 presence is advertised separately for stage1 and
stage2, the expectation is that in practice both stages will either
support or not support it. Therefore, we combine both into a single
capability, allowing us to simplify the implementation. KVM requires
support in both stages in order to use LPA2 since the same library is
used for hyp stage 1 and guest stage 2 pgtables.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>