Per imx23-clock.yaml and imx28-clock.yaml, fsl,clkctrl is not a valid
compatible string.

Remove it to fix the following dt-schema warning:

imx28-evk.dtb: clkctrl@80040000: compatible: ['fsl,imx28-clkctrl', 'fsl,clkctrl'] is too long
	from schema $id: http://devicetree.org/schemas/clock/imx28-clock.yaml#Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>