bus_dma_dmar_set_buswide(9): KPI to indicate that the whole dmar
context should share page tables.

Practically it means that dma requests from any device on the bus are
translated according to the entries loaded for the bus:0:0 device.
KPI requires that the slot and function of the device be 0:0, and that
no tags for other devices on the bus were used.

The intended use are NTBs which pass TLPs from the downstream to the
host with slot:func of the downstream originator.

Reviewed and tested by: mav
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D22434