pinctrl: mcp23s08: add module license

Fix the following build warning:

WARNING: modpost: missing MODULE_LICENSE() in
drivers/pinctrl/pinctrl-mcp23s08.o
see include/linux/module.h for more information

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Link: https://lore.kernel.org/r/20200417092125.12513-1-yanaijie@huawei.com
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/pinctrl-mcp23s08.c

@@ -658,3 +658,4 @@ int mcp23s08_probe_one(struct mcp23s08 *mcp, struct device *dev,
 	return ret;
 }
 EXPORT_SYMBOL_GPL(mcp23s08_probe_one);
+MODULE_LICENSE("GPL");