ab8500-bm: Add usb power path support

AB8540 supports power path function in USB charging mode for fast
power up with dead and weak battery, and it could extend
the battery age.

When USB charging starts, if the Vbattrue is below than SW cut off
voltage, power path and pre-charge should be enabled. If Vbattrue
is higher than SW cut off voltage, power path and pre-charge should
be disabled. This is to make sure full current to battery charge.
At the end of charge, power path should be enable again to reduce
charging the battery again.

Signed-off-by: Lee Jones <lee.jones@linaro.org>