arm64: a37xx: pinctrl: Implement gpio_request_enable for gpio functionality

To automatically enable GPIO functionality of some MPP pin, it is required
to implement .gpio_request_enable and .gpio_disable_free callbacks in
pinctrl driver and set .request and .rfree callbacks in GPIO driver to
pinctrl_gpio_request / pinctrl_gpio_free functions.

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