KVM: arm64: Use generic sanitisation for ID_AA64PFR0_EL1

KVM allows userspace to write to the CSV2 and CSV3 fields of
ID_AA64PFR0_EL1 so long as it doesn't over-promise on the
Spectre/Meltdown mitigation state. Switch over to the new way of the
world for screening user writes. Leave the old plumbing in place until
we actually start handling ID register reads from the VM-wide values.

Signed-off-by: Jing Zhang <jingzhangos@google.com>
Link: https://lore.kernel.org/r/20230609190054.1542113-10-oliver.upton@linux.dev


[Oliver: split from monster patch, added commit description]
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>