o Move iommu_set_buswide_ctx, iommu_is_buswide_ctx to

the generic iommu busdma backend;

o Move bus_dma_iommu_set_buswide, bus_dma_iommu_load_ident

prototypes to iommu.h.

Reviewed by: kib
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D25866