ASoC: dt-bindings: add max98088 audio codec

This patch adds the bindings for maxim max98088/9 audio codec.

Signed-off-by: Andreas Färber <afaerber@suse.de>
[m.felsch@pengutronix.de: adapt commit message]
[m.felsch@pengutronix.de: adapt formatting]
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>