gpio: aspeed: add MODULE_LICENSE()

The build complains about missing MODULE_LICENSE() in
the Aspeed GPIO driver. The license is evident from the
file header, put in "GPL".

Reported-by: Stephen Rothwell <[e-mail]>
Cc: Alistair Popple <[e-mail]>
Cc: Jeremy Kerr <[e-mail]>
Cc: Andrew Jeffery <[e-mail]>
Acked-by: Joel Stanley <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>

drivers/gpio/gpio-aspeed.c

···
 module_platform_driver_probe(aspeed_gpio_driver, aspeed_gpio_probe);
 
 MODULE_DESCRIPTION("Aspeed GPIO Driver");
+MODULE_LICENSE("GPL");