dt-bindings: display: Add Synopsys DW MIPI DSI host controller

This patch adds documentation of device tree bindings for the
Synopsys DesignWare MIPI DSI host controller.

Signed-off-by: Philippe CORNU <philippe.cornu@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1500277223-29553-4-git-send-email-philippe.cornu@st.com