ARM: dts: imx23/28: Remove unneeded "fsl,mxs-gpio"

According to gpio-mxs.yaml, only "fsl,imx23-gpio" or "fsl,imx28-gpio"
are valid compatible strings.

This fixes the following dt-schema warnings:

imx23-evk.dtb: pinctrl@80018000: gpio@0:compatible: ['fsl,imx23-gpio', 'fsl,mxs-gpio'] is too long
	from schema $id: http://devicetree.org/schemas/gpio/gpio-mxs.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>