dt-bindings: mipi-dsi: Add info about peripherals with non-DSI control bus

Add a section that describes dt-bindings for peripherals that support
MIPI DSI, but have a different bus as the primary control bus, or no
control bus at all. Add an example for a peripheral with a non-DSI
control bus.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Philippe Cornu <philippe.cornu@st.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180709090751.10221-2-architt@codeaurora.org