regmap: Add file patterns for regmap 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: NMark Brown <broonie@kernel.org>

MAINTAINERS

@@ -9318,6 +9318,7 @@ M:	Mark Brown <broonie@kernel.org>
 L:	linux-kernel@vger.kernel.org
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/broonie/regmap.git
 S:	Supported
+F:	Documentation/devicetree/bindings/regmap/
 F:	drivers/base/regmap/
 F:	include/linux/regmap.h