KVM: arm64: Add missing field descriptor for MDCR_EL2

It's not currently used. Added for completeness.

No functional change intended.

Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20211010145636.1950948-6-tabba@google.com