MAINTAINERS: Add file patterns for mips 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: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13340/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

MAINTAINERS

@@ -7270,6 +7270,7 @@ W:	http://www.linux-mips.org/
 T:	git git://git.linux-mips.org/pub/scm/ralf/linux.git
 Q:	http://patchwork.linux-mips.org/project/linux-mips/list/
 S:	Supported
+F:	Documentation/devicetree/bindings/mips/
 F:	Documentation/mips/
 F:	arch/mips/