vtscmi: Add a virtio-scmi driver
Add a new virtio backend to support SCMI VirtIO devices (type 32) as
defined by the VirtIO specification since version v1.2.
https://docs.oasis-open.org/virtio/virtio/v1.2/cs01/virtio-v1.2-cs01.pdf
Reviewed by: andrew, bryanv
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D43047