Add BERI-specific virtio mmio "platform" device.
This device provides a connector to Altera PIO allowing us
to interrupt software-implemented virtio mmio backend.

Sponsored by:	DARPA, AFRL


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274714 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f