iommu/arm-smmu-v3-kvm: Respect the 'disabled' status in device node

If the SMMU device node is in a 'disabled' status, the current code does
not ignore the SMMU device, and consequently, it will lead to a pKVM
hypervisor initialization failure.

This patch checks the device node status and filters out any disabled
SMMU devices. Finally, the 'disabled' SMMU device will not be accounted
and avoid the initialization failure.

Signed-off-by: Leo Yan <leo.yan@arm.com>