dt-bindings: display: bridge: Add documentation for LT9611

Lontium LT9611 is a DSI to HDMI bridge which supports 2 DSI ports
and I2S port as input and one HDMI port as output

Reviewed-by: Rob Herring <robh@kernel.org>
Tested-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200723163442.1280414-3-vkoul@kernel.org