Start splitting-out the Intel DMAR busdma backend to a generic place,
so it can be used on other IOMMU systems.

Provide MI iommu_unit, iommu_domain and iommu_ctx structs in sys/iommu.h;
use them as a first member of MD dmar_unit, dmar_domain and dmar_ctx.

Change the namespace in DMAR backend: use iommu_ prefix instead of dmar_.

Move some macroses and function prototypes to sys/iommu.h.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D25574