gpio: ab8500: fix MODULE_ALIAS for ab8500

Since 43cc71ee

 (platform: prefix MODALIAS
with "platform:"), the platform modalias is prefixed with "platform:".

This patch changes the MODULE_ALIAS to "platform:ab8500-gpio".

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>