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>

Documentation/devicetree/bindings/sound/max98090.txt

@@ -16,6 +16,8 @@ Optional properties:
 
 - clock-names: Should be "mclk"
 
+- #sound-dai-cells : should be 0.
+
 - maxim,dmic-freq: Frequency at which to clock DMIC
 
 - maxim,micbias: Micbias voltage applies to the analog mic, valid voltages value are: