iommu_gas: add iommu_gas_remove()

The function removes the range of addresses from GAS. Right now it is
unused.

The initial version of the patch was extracted from the stalled work for
integration of DMAR and bhyve, see D25672. Having the function in the
main tree would allow it to co-evolve with other active changes to the
IOMMU driver.

Requested by: alc
Reviewed by: alc, dougm
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D36001