pinctrl: mediatek: correct debounce time unit in mtk_gpio_set_debounce

The debounce time unit for gpio_chip.set_debounce is us but
mtk_gpio_set_debounce regard it as ms.
Fix this by correct debounce time array dbnc_arr so it can find correct
debounce setting. Debounce time for first debounce setting is 500us,
correct this as well.

While I'm at it, also change the debounce time array name to
"debounce_time" for readability.

Cc: stable@vger.kernel.org
Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>