MAINTAINERS: Cover Arm SMMU DT bindings

We'd like to be aware of proposed DT binding changes even when they
don't directly touch the drivers themselves.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

MAINTAINERS

@@ -1426,6 +1426,7 @@ M:	Will Deacon <will@kernel.org>
 R:	Robin Murphy <robin.murphy@arm.com>
 L:	linux-arm-kernel@lists.infradead.org (moderated for non-subscribers)
 S:	Maintained
+F:	Documentation/devicetree/bindings/iommu/arm,smmu*
 F:	drivers/iommu/arm-smmu*
 F:	drivers/iommu/io-pgtable-arm.c
 F:	drivers/iommu/io-pgtable-arm-v7s.c