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

drivers/pinctrl/freescale/Kconfig

 # SPDX-License-Identifier: GPL-2.0-only
 config PINCTRL_IMX
-	bool
+	tristate
+	depends on OF
 	select GENERIC_PINCTRL_GROUPS
 	select GENERIC_PINMUX_FUNCTIONS
 	select GENERIC_PINCONF

drivers/pinctrl/freescale/pinctrl-imx.c

@@ -11,6 +11,7 @@
 #include <linux/init.h>
 #include <linux/io.h>
 #include <linux/mfd/syscon.h>
+#include <linux/module.h>
 #include <linux/of.h>
 #include <linux/of_device.h>
 #include <linux/of_address.h>
@@ -898,3 +899,7 @@ const struct dev_pm_ops imx_pinctrl_pm_ops = {
 					imx_pinctrl_resume)
 };
 EXPORT_SYMBOL_GPL(imx_pinctrl_pm_ops);
+
+MODULE_AUTHOR("Dong Aisheng <aisheng.dong@nxp.com>");
+MODULE_DESCRIPTION("NXP i.MX common pinctrl driver");
+MODULE_LICENSE("GPL v2");