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>