dt-bindings: display: bridge: Add documentation for Toshiba tc358768

TC358768/TC358778 is a Parallel RGB to MIPI DSI bridge.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200131111553.472-2-peter.ujfalusi@ti.com