ASoC: Add file patterns for sound 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>
Signed-off-by: Mark Brown <broonie@kernel.org>

MAINTAINERS

@@ -10702,6 +10702,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/broonie/sound.git
 L:	alsa-devel@alsa-project.org (moderated for non-subscribers)
 W:	http://alsa-project.org/main/index.php/ASoC
 S:	Supported
+F:	Documentation/devicetree/bindings/sound/
 F:	Documentation/sound/alsa/soc/
 F:	sound/soc/
 F:	include/sound/soc*