devices: report iommu data for the device in the dev. sysctl tree

Show the iommu unit' device name/unit which translates the device'
requests, if any, as parsed out from the BIOS tables.
Also show the rid value for the device.

Sponsored by: Advanced Micro Devices (AMD)
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D47098