MAINTAINERS: add Documentation files to dmaengine entry

We were missing the Documentation files in the entry so add it now. While at
it also remove comment on slave-dma for the tree

Signed-off-by: Vinod Koul <vinod.koul@intel.com>

MAINTAINERS

@@ -3080,7 +3080,8 @@ Q:	https://patchwork.kernel.org/project/linux-dmaengine/list/
 S:	Maintained
 F:	drivers/dma/
 F:	include/linux/dma*
-T:	git git://git.infradead.org/users/vkoul/slave-dma.git (slave-dma)
+F:	Documentation/dmaengine/
+T:	git git://git.infradead.org/users/vkoul/slave-dma.git
 
 DME1737 HARDWARE MONITOR DRIVER
 M:	Juerg Haefliger <juergh@gmail.com>