ARM: dts: imx6sx: remove fallback compatible string fsl,imx28-lcdif

ARM: dts: imx6sx: remove fallback compatible string fsl,imx28-lcdif

The i.MX6SX LCDIF is not fully compatible with the i.MX28 LCDIF. The
i.MX6SX controller provides additional overlay registers (AS_CTRL) which
are not present on i.MX28.

Linux has supported the dedicated compatible string since commit
45d59d704080 ("drm: Add new driver for MXSFB controller").

Other known DT users such as U-Boot and Barebox already support
"fsl,imx6sx-lcdif", so removing the fallback compatible string is low risk
since this device is used for display output only.

Fix the following CHECK_DTB warning:
/arch/arm/boot/dts/nxp/imx/imx6sx-nitrogen6sx.dtb: lcdif@2220000 (fsl,imx6sx-lcdif): compatible: 'oneOf' conditional failed, one must be fixed:
        ['fsl,imx6sx-lcdif', 'fsl,imx28-lcdif'] is too long

Signed-off-by: Frank Li <Frank.Li@nxp.com>