mfd: 88pm860x: Move initilization code

Move probe() and other functions from 88pm860x-i2c.c to 88pm860x-core.c.
Since it could benefit to handle DT information.

Signed-off-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>