arm64: cpufeature: Handle NV_frac as a synonym of NV2

arm64: cpufeature: Handle NV_frac as a synonym of NV2

With ARMv9.5, an implementation supporting Nested Virtualization
is allowed to only support NV2, and to avoid supporting the old
(and useless) ARMv8.3 variant.

This is indicated by ID_AA64MMFR2_EL1.NV being 0 (as if NV wasn't
implemented) and ID_AA64MMFR4_EL1.NV_frac being 1 (indicating that
NV2 is actually supported).

Given that KVM only deals with NV2 and refuses to use the old NV,
detecting NV2 or NV_frac is what we need to enable it.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20250220134907.554085-2-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>