gpio: ath79: add missing MODULE_DESCRIPTION/LICENSE

This change resolves a new compile-time warning
when built as a loadable module:

WARNING: modpost: missing MODULE_LICENSE() in drivers/gpio/gpio-ath79.o
see include/linux/module.h for more information

This adds the license as "GPL v2", which matches the header of the file.

MODULE_DESCRIPTION is also added.

Signed-off-by: Jesse Chan <jc@linux.com>
Acked-by: Alban Bedel <albeu@free.fr>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>