dt-bindings: mipi-dsi: Add dual-channel DSI related info

Add binding info for peripherals that support dual-channel DSI. Add
corresponding optional bindings for DSI host controllers that may
be configured in this mode. Add an example of an I2C controlled
device operating in dual-channel DSI mode.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709090751.10221-3-architt@codeaurora.org