KVM: arm64: Describe RES0/RES1 bits of MDCR_EL2

Add support for sanitising MDCR_EL2 and describe the RES0/RES1 bits
according to the feature set exposed to the VM.

Reviewed-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20241025182354.3364124-6-oliver.upton@linux.dev


Signed-off-by: Oliver Upton <oliver.upton@linux.dev>