Add a dmardump utility to dump the VT-d context tables.

This tool parses the ACPI DMAR table looking for DMA remapping devices.
For each device it walks the root table and any context tables
referenced to display mapping info for PCI devices.

Note that acpidump -t already parses the info in the ACPI DMAR tables
directly. This tool examines some of the data structures the DMAR
remapping engines use to translate DMA requests.

Reviewed by: kib, grehan
MFC after: 1 month
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D7444