ASoC: max98090: Add #sound-dai-cells property documentation

MAX98090 CODEC supports 1 input DAI, adding the #sound-dai-cells property
helps to reference the DAI in DT in a standard way.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mark Brown <broonie@kernel.org>