mfd: mt6397: Extract IRQ related code from core driver

In order to support different types of irq design, we decide to add
separate irq drivers for different design and keep mt6397 mfd core
simple and reusable to all generations of PMICs so far.

Signed-off-by: Hsin-Hsiung Wang <hsin-hsiung.wang@mediatek.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>