drm: bridge: it66121: Add support for the IT6610

Add support for the IT6610 HDMI encoder.

The hardware is very similar, and therefore the driver did not require
too many changes. Some bits are only available on the IT66121, and
vice-versa. Also, the IT6610 requires specific polarities on the DE and
pixel lines.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20221214130131.12962-1-paul@crapouillou.net