pinctrl: ingenic: make function ingenic_pinctrl_probe static

The function ingenic_pinctrl_probe is local to the source and does
not need to be in global scope, so make it static.

Cleans up sparse warnings
symbol 'ingenic_pinctrl_probe' was not declared. Should it be static?

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