mfd: mt6397: IRQ domain should initialize before mfd_add_devices()

Some sub driver like RTC module need irq domain from parent to create
irq mapping when driver initialize. so move mt6397_irq_init() before
mfd_add_devices().

Acked-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Henry Chen <henryc.chen@mediatek.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>