MAINTAINERS: Add file patterns for mtd device tree bindings

Submitters of device tree binding documentation may forget to CC
the subsystem maintainer if this is missing.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Author: geertu

MAINTAINERS

@@ -7457,6 +7457,7 @@ Q:	http://patchwork.ozlabs.org/project/linux-mtd/list/
 T:	git git://git.infradead.org/linux-mtd.git
 T:	git git://git.infradead.org/l2-mtd.git
 S:	Maintained
+F:	Documentation/devicetree/bindings/mtd/
 F:	drivers/mtd/
 F:	include/linux/mtd/
 F:	include/uapi/mtd/