gpio: mvebu_gpio: Add .request and .rfree methods for Armada 38x

To use particular pin GPIO, it needs to be first switched to GPIO by
pinctrl. Use pinctrl_gpio_request() and pinctrl_gpio_free() for this
purpose.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>