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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reported-by: Randy Dunlap <[e-mail]>
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/pinctrl/pinctrl-mcp23s08.c

···
 	return ret;
 }
 EXPORT_SYMBOL_GPL(mcp23s08_probe_one);
+MODULE_LICENSE("GPL");