dt-bindings: gpio: Move fsl,mxs-pinctrl.txt into gpio-mxs.yaml

dt-bindings: gpio: Move fsl,mxs-pinctrl.txt into gpio-mxs.yaml

Move mxs-pinctrl part into gpio-mxs.yaml and add pinctrl examples to fix
below CHECK_DTB warning:

arch/arm/boot/dts/nxp/mxs/imx28-xea.dtb: pinctrl@80018000 (fsl,imx28-pinctrl):
   'auart0-2pins@0', 'auart0@0',  ... 'usb1@1' do not match any of the regexes: 'gpio@[0-9]+$', 'pinctrl-[0-9]+'

Signed-off-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250820164946.3782702-1-Frank.Li@nxp.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>