dt-bindings: display: bridge: it66121: Add compatible string for IT6610

Add a new ite,it6610 compatible string to the IT66121 binding
documentation, since the two chips are very similar.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221214125821.12489-2-paul@crapouillou.net