MAINTAINERS: add entry for dma mapping helpers

This code has been spread between getting in through arch trees, the iommu
tree, -mm and the drivers tree.  There will be a lot of work in this area,
including consolidating various arch implementations into more common
code, so ensure we have a proper git tree that facilitates cooperation with
the architecture maintainers.

Signed-off-by: Christoph Hellwig <hch@lst.de>