mfd: mt6397: Trim probe function to support different chips more cleanly

Add new struct members for mfd-cells and irq initial function, so we can
call devm_mfd_add_devices() only once.

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