MAINTAINERS: add the iommu list for swiotlb and xen-swiotlb

All other discussions related to the dma mapping interfaces are on the
iommu list, so let's make it the official list for swiotlb and the
second list for xen-swiotlb.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>