ASoC: doc: simple-card: Add pin-switches property

The simple-audio-card,pin-switches property can contain the list of
widget names for which pin switches must be created.
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/simple-card.txt

@@ -24,6 +24,8 @@ Optional properties:
 					  a microphone is attached.
 - simple-audio-card,aux-devs		: List of phandles pointing to auxiliary devices, such
 					  as amplifiers, to be added to the sound card.
+- simple-audio-card,pin-switches	: List of strings containing the widget names for
+					  which pin switches must be created.
 
 Optional subnodes: