Input: of_touchscreen - add MODULE_LICENSE

The lack of the MODULE_LICENSE tag can lead to a warning here:

WARNING: modpost: missing MODULE_LICENSE() in drivers/input/touchscreen/of_touchscreen.o

I'm adding a license and description tag, but no MODULE_AUTHOR()
as this file is a collection of standalone helper functions that
were all added by different developers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>