pinctrl/amd: make functions amd_gpio_suspend and amd_gpio_resume static

The functions amd_gpio_suspend and amd_gpio_resume are local to the
source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
symbol 'amd_gpio_suspend' was not declared. Should it be static?
symbol 'amd_gpio_resume' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Daniel Drake <drake@endlessm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>