mfd: mt6397: Add MT6323 LED support into MT6397 driver

Add compatible string as "mt6323-led" that will make
the OF core spawn child devices for the LED subnode
of that MT6323 MFD device.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>