o Add machine/iommu.h and include MD iommu headers from it,

so we don't ifdef for every arch in busdma_iommu.c;

o No need to include specialreg.h for x86, remove it.

Requested by: andrew
Reviewed by: kib
Sponsored by: DARPA/AFRL
Differential Revision: https://reviews.freebsd.org/D25957