dt-bindings: display: bridge: it66121: Add audio support

Update the ITE bridge HDMI it66121 bindings in order to
support audio.

Signed-off-by: Nicolas Belin <nbelin@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220316135733.173950-2-nbelin@baylibre.com