pinctrl: mxs: Remove undocumented 'fsl,mxs-gpio' property

pinctrl: mxs: Remove undocumented 'fsl,mxs-gpio' property

The 'fsl,mxs-gpio' property is not documented in gpio-mxs.yaml, but
the imx23 and imx28 dtsi describe the gpios as:

compatible = "fsl,imx28-gpio", "fsl,mxs-gpio";

This gives schema warnings like:

imx28-cfa10037.dtb: pinctrl@80018000: gpio@0:compatible: ['fsl,imx28-gpio', 'fsl,mxs-gpio'] is too long
	from schema $id: http://devicetree.org/schemas/gpio/gpio-mxs.yaml#

"fsl,mxs-gpio" is only used inside pinctrl-mxs, but can be removed if
the compatible check is done against fsl,imx23-gpio and fsl,imx28-gpio.

Introduce is_mxs_gpio() and remove the need for "fsl,mxs-gpio".

Tested on a imx28-evk.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Link: https://lore.kernel.org/r/20230928134321.438547-1-festevam@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>