MAINTAINERS: Add file patterns for iommu device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.
Signed-off-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: iommu@lists.linux-foundation.org
Signed-off-by: NJoerg Roedel <jroedel@suse.de>

MAINTAINERS

@@ -6145,6 +6145,7 @@ M:	Joerg Roedel <joro@8bytes.org>
 L:	iommu@lists.linux-foundation.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu.git
 S:	Maintained
+F:	Documentation/devicetree/bindings/iommu/
 F:	drivers/iommu/
 
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