ASoC: dt-bindings: mt8183: add compatible string for using max98357b

Machines with max98357b should use the compatible string
"mt8183-mt6358-ts3a227-max98357b".
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200720012559.906088-3-tzungbi@google.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/mt8183-mt6358-ts3a227-max98357.txt

@@ -2,6 +2,7 @@ MT8183 with MT6358, TS3A227, MAX98357, and RT1015 CODECS
 
 Required properties:
 - compatible : "mediatek,mt8183_mt6358_ts3a227_max98357" for MAX98357A codec
+               "mediatek,mt8183_mt6358_ts3a227_max98357b" for MAX98357B codec
                "mediatek,mt8183_mt6358_ts3a227_rt1015" for RT1015 codec
 - mediatek,platform: the phandle of MT8183 ASoC platform