ASoC: cs4234: Add dtschema binding document

Document the bindings for the CS4234 ASoC codec driver.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20200928111821.26967-1-rf@opensource.cirrus.com


Signed-off-by: Mark Brown <broonie@kernel.org>