KVM: arm64: Allow activating realms

Add the ioctl to activate a realm and set the static branch to enable
access to the realm functionality if the RMM is detected.

Signed-off-by: Steven Price <steven.price@arm.com>
Reviewed-by: Gavin Shan <gshan@redhat.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>