video: mxsfb: add back imx6ul/imx6ull support

Because of commit bf947d2a ("imx6ul: synchronise device tree with
linux"), the compatible property of lcdif in imx6ul.dtsi went from

compatible = "fsl,imx6ul-lcdif", "fsl,imx28-lcdif";

to

compatible = "fsl,imx6ul-lcdif", "fsl,imx6sx-lcdif";

without updating the mxsfb driver to match that change.

Add "fsl,imx6sx-lcdif" as a compatible id to fix that.

Fixes: bf947d2a ("imx6ul: synchronise device tree with linux")
Signed-off-by: Sébastien Szymanski <sebastien.szymanski@armadeus.com>