MAINTAINERS: Add file patterns for mmc 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>
Signed-off-by: NUlf Hansson <ulf.hansson@linaro.org>

MAINTAINERS

@@ -7744,6 +7744,7 @@ M:	Ulf Hansson <ulf.hansson@linaro.org>
 L:	linux-mmc@vger.kernel.org
 T:	git git://git.linaro.org/people/ulf.hansson/mmc.git
 S:	Maintained
+F:	Documentation/devicetree/bindings/mmc/
 F:	drivers/mmc/
 F:	include/linux/mmc/
 F:	include/uapi/linux/mmc/