regulator: 88pm860x: Add pre-regulator support for 88pm860x regulator

Pre-regulator of 88pm8606 is mainly for support charging based on vbus,
it needs to be enabled for charging battery, and will be disabled in
some exception condition like over-temp.
Add the pre-regulator support for 88pm860x regulator driver.

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