scmi: Add an SCMI VirtIO transport driver

[cristian.marussi_arm.com]

Summary

Add an SCMI transport driver based on the virtio-scmi backend.

Sponsored by: Arm Ltd

[bryanv]

Just as a general comment - some of the device_printf() look potentially spammy and may be better as either guarded by bootverbose or debug macros.

sys/dev/firmware/arm/scmi_virtio.c

93

Nit: M_WAITOK means this cannot fail. May be slightly clearer to use mallocarray() instead.

[cristian.marussi_arm.com]

Using mallocarray and dropping empty interface method

Using mallocarray and avoding to check the retval since called as M_WAITOK.
Dropping scmi_virtio_detach() driver method since it just calls into the parent
scmi_attach()