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 <sfr@canb.auug.org.au>
Cc: Alistair Popple <alistair@popple.id.au>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: Andrew Jeffery <andrew@aj.id.au>
Acked-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>