pinctrl: imx: Support building i.MX pinctrl core driver as module

pinctrl: imx: Support building i.MX pinctrl core driver as module

Change PINCTRL_IMX to tristate to support loadable module build.

And i.MX common pinctrl driver should depend on CONFIG_OF to make sure
no build error when i.MX common pinctrl driver is enabled for different
architectures without CONFIG_OF.

Also add module author, description and license.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Link: https://lore.kernel.org/r/1599552721-24872-3-git-send-email-Anson.Huang@nxp.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>