MAINTAINERS: Update VT-d entry for drivers/pci -> drivers/iommu move

Commit 166e9278 ("x86/ia64: intel-iommu: move to drivers/iommu/")
moved the VT-d driver to drivers/iommu, but left the "F:" line in
MAINTAINERS pointing to drivers/pci, which breaks scripts/get_maintainer.pl.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>