MTK EINT does not support generating interrupt on both edges.
Emulate this by changing edge polarity while enable irq,
set types and interrupt handling. This follows an example of
drivers/gpio/gpio-mxc.c.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Signed-off-by: Chaotian Jing <chaotian.jing@mediatek.com>
Acked-by: Hongzhou Yang <hongzhou.yang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>