MAINTAINERS: Add related headers to IOMMU section

The linux/iommu.h and linux/iova.h headers belong to the IOMMU subsystem
but scripts/get_maintainers.pl currently fails to assign them because
they aren't listed in MAINTAINERS.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>