MAINTAINERS: use the iommu list for the dma-mapping subsystem

Maintaining a subsystem with linux-kernel as the main list is painful
as it has way to much traffic.  On the other hand the dma-mapping
subsystem is small enough that a list on its own would be silly.
So use the list for the closes subsystem instead instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Joerg Roedel <jroedel@suse.de>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>