dt-bindings: pinctrl: imx8ulp: Add pinctrl binding

Add pinctrl binding doc for i.MX8ULP

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Link: https://lore.kernel.org/r/20210607061041.2654568-1-ping.bai@nxp.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>