dt-bindings: display: dw_mipi_dsi.txt: convert to yaml

This converts the Synopsis MIPI DSI binding documentation to yaml and
should be quite straightforward. I've added a missing ref clk and also
added Philippe as maintainer b/c he's the original txt author following
the algorithm provided in Message-ID 20200420175909.GA5810@ravnborg.org.

Cc: Philippe CORNU <philippe.cornu@st.com>
Cc: devicetree@vger.kernel.org
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200423100058.1734009-1-adrian.ratiu@collabora.com