ASoC: dt-bindings: mt8183: add missing update

Headset codec is optional.  Add missing update to DT binding document.

Fixes: a962a809 ("ASoC: mediatek: mt8183: make headset codec optional")
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20190920112320.166052-1-tzungbi@google.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -2,9 +2,11 @@ MT8183 with MT6358, TS3A227 and MAX98357 CODECS
 
 Required properties:
 - compatible : "mediatek,mt8183_mt6358_ts3a227_max98357"
-- mediatek,headset-codec: the phandles of ts3a227 codecs
 - mediatek,platform: the phandle of MT8183 ASoC platform
 
+Optional properties:
+- mediatek,headset-codec: the phandles of ts3a227 codecs
+
 Example:
 
 	sound {