gpio: mvebu: Set free callback for gpio_chip

We call pinctrl_request_gpio() in request callback, thus we need to call
pinctrl_free_gpio() in free callback.

Both mvebu_gpio_request() and mvebu_gpio_free() are not referenced outside of
this file, make them static.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>