KVM: arm64: Make ID_AA64PFR1_EL1.RAS_frac writable

KVM: arm64: Make ID_AA64PFR1_EL1.RAS_frac writable

Allow userspace to write to RAS_frac, under the condition that
the host supports RASv1p1 with RAS_frac==1. Other configurations
will result in RAS_frac being exposed as 0, and therefore implicitly
not writable.

To avoid the clutter, the ID_AA64PFR1_EL1 sanitisation is moved to
its own function.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Link: https://lore.kernel.org/r/20250817202158.395078-6-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>