KVM: arm64: Add ARM64_HAS_LPA2 CPU capability

Expose FEAT_LPA2 as a capability so that we can take advantage of
alternatives patching in both the kernel and 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, for the case where KVM is present,
we combine both into a single capability, allowing us to simplify the
implementation. For the case where KVM is not present, we only care
about stage1.

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