drm/panel: Add generic DSI display controller YAML bindings

drm/panel: Add generic DSI display controller YAML bindings

This adds a starting point for processing and defining generic
bindings used by DSI display controllers and panels attached to
the virtual DSI ports.

Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: devicetree@vger.kernel.org
Suggested-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191128090726.51107-1-linus.walleij@linaro.org
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>