dt-bindings: display: mxsfb: Add and fix compatible strings

Additional compatible strings have been added in DT source for the
i.MX6SL, i.MX6SLL, i.MX6UL and i.MX7D without updating the bindings.
Most of the upstream DT sources use the fsl,imx28-lcdif compatible
string, which mostly predates the realization that the LCDIF in the
i.MX6 and newer SoCs have extra features compared to the i.MX28.

Update the bindings to add the missing compatible strings, with the
correct fallback values. This fails to validate some of the upstream DT
sources. Instead of adding the incorrect compatible fallback to the
binding, the sources should be updated 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/20210115222304.5427-3-laurent.pinchart@ideasonboard.com