dt-bindings: display: mxsfb: Convert binding to YAML

Convert the mxsfb binding to YAML. The deprecated binding is dropped, as
neither the DT sources nor the driver support it anymore. The converted
binding is named fsl,lcdif.yaml to match the usual bindings naming
scheme.

The compatible strings are messy, and DT sources use different kinds of
combination of documented and undocumented values. Keep it simple for
now, and update the example to make it valid. Aligning the binding with
the existing DT sources will be performed separately.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115222304.5427-2-laurent.pinchart@ideasonboard.com