mfd: mt6397: Create irq mappings in mfd core driver

The core driver should create and manage irq mappings instead of
leaf drivers. This patch change to pass irq domain to
devm_mfd_add_devices() and it will create mapping for irq resources
automatically. And remove irq mapping in rtc driver since this has
been done in core driver.

Signed-off-by: Chen Zhong <chen.zhong@mediatek.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>