ASoC: cs42l51: add Device Tree binding to cs42l51

This commit adds a trivial Device Tree binding to the I2C-based
cs42l51 sound codec, so that it can be used from Device Tree based
platforms.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Mark Brown <broonie@linaro.org>