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 <[e-mail]>
Acked-by: Robin Murphy <[e-mail]>
Acked-by: Joerg Roedel <[e-mail]>
Acked-by: Marek Szyprowski <[e-mail]>

MAINTAINERS

···
 M:	Christoph Hellwig <[e-mail]>
 M:	Marek Szyprowski <[e-mail]>
 R:	Robin Murphy <[e-mail]>
-L:	[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