dt-bindings: Added device tree binding for max98390

Add DT binding of max98390 amplifier driver.

Signed-off-by: Steve Lee <steves.lee@maximintegrated.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200602084337.22116-1-steves.lee@maximintegrated.com


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