Allow virtio_mmio to attach to ofwbus. Qemu places these here on at least
the AArch64 virtual platform with the Linaro UEFI.

Sponsored by:	The FreeBSD Foundation


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276358 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f