dt-bindings: display: mxsfb: Add compatible for i.MX8MM

NXP's i.MX8MM has an LCDIF as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115222304.5427-4-laurent.pinchart@ideasonboard.com