ASoC: simple-card-utils: add asoc_simple_parse_pin_switches()

This function is a helper that permits to create pin switch controls for
a list of widgets whose names are listed in the PREFIX "pin-switches"
devicetree property.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Mark Brown <broonie@kernel.org>