ARM: dts: imx6ul: fix lcdif node compatible

ARM: dts: imx6ul: fix lcdif node compatible

In yaml binding "fsl,imx6ul-lcdif" is listed as compatible to imx6sx-lcdif,
but not imx28-lcdif. Change the list accordingly. Fixes the
dt_binding_check warning:
lcdif@21c8000: compatible: 'oneOf' conditional failed, one must be fixed:
['fsl,imx6ul-lcdif', 'fsl,imx28-lcdif'] is too long
Additional items are not allowed ('fsl,imx28-lcdif' was unexpected)
'fsl,imx6ul-lcdif' is not one of ['fsl,imx23-lcdif', 'fsl,imx28-lcdif',
'fsl,imx6sx-lcdif']
'fsl,imx6sx-lcdif' was expected

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>