KVM: arm64: nv: Allow userspace to de-feature stage-2 TGRANs

KVM: arm64: nv: Allow userspace to de-feature stage-2 TGRANs

KVM advertises the stage-2 TGRAN fields as writable to userspace but
prevents any modification for NV-enabled VMs. Update the special-cased
sanitization to permit de-featuring a particular TGRAN without allowing
the legacy value which refers to the stage-1 field for support.

Reported-by: Itaru Kitayama <itaru.kitayama@linux.dev>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>