KVM: selftests: Comment newly defined aarch64 ID registers

All otherwise unspecified aarch64 ID registers should be read as zero so
we cover the whole ID register space in the get-reg-list test but we've
added comments for those that have been named. Add comments for
ID_AA64PFR2_EL1, ID_AA64SMFR0_EL1, ID_AA64ISAR2_EL1, ID_AA64MMFR3_EL1
and ID_AA64MMFR4_EL1 which have been defined since the comments were
added so someone looking for them will see that they are covered.

Signed-off-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230210-kvm-arm64-getreg-comments-v1-1-a16c73be5ab4@kernel.org