pinctrl: mt8173: set GPIO16 to usb iddig mode

the default mode of GPIO16 pin is gpio, when set EINT16 to
IRQ_TYPE_LEVEL_HIGH, no interrupt is triggered, it can be
fixed when set its default mode as usb iddig.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Acked-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>