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 <[e-mail]>

MAINTAINERS

···
 F:	net/bluetooth/
 F:	include/net/bluetooth/
 
+DMA MAPPING HELPERS
+M:	Christoph Hellwig <[e-mail]>
+M:	Marek Szyprowski <[e-mail]>
+R:	Robin Murphy <[e-mail]>
+L:	[e-mail]
+T:	git git://git.infradead.org/users/hch/dma-mapping.git
+W:	http://git.infradead.org/users/hch/dma-mapping.git
+S:	Supported
+F:	lib/dma-debug.c
+F:	lib/dma-noop.c
+F:	lib/dma-virt.c
+F:	drivers/base/dma-mapping.c
+F:	drivers/base/dma-coherent.c
+F:	include/linux/dma-mapping.h
+
 BONDING DRIVER
 M:	Jay Vosburgh <[e-mail]>
 M:	Veaceslav Falico <[e-mail]>