dt-bindings: display: bridge: Add documentation for LT8912B

Lontium LT8912B is a DSI to HDMI bridge.

Signed-off-by: Adrien Grassein <adrien.grassein@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210326121955.1266230-2-adrien.grassein@gmail.com