drm: bridge: add it66121 driver

This commit is a simple driver for bridge HMDI it66121.
The input format is RBG and there is no color conversion.
Audio, HDCP and CEC are not supported yet.

Signed-off-by: Phong LE <ple@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210419071223.2673533-3-narmstrong@baylibre.com