pinctrl: mediatek: Add gpio_request_enable support

Implement the .gpio_request_enable() callbacks in struct pinmux_ops
in mediatek pinctrl driver. Make sure that when gpio_request is called,
GPIO on the pin is enabled.

Signed-off-by: Biao Huang <biao.huang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>