charger: bq25180: Add threshold voltage for precharge

Charging a battery has three phases (compare Figure 8-2 in
https://www.ti.com/lit/ds/symlink/bq25180.pdf

)

First is a pre-charge phase, then a constant current, then a constant
voltage phase.
During the pre-charge phase, the battery is only charged with a very low
current, to not damage the cells, because they are below a threshold,
that is considered healthy for the battery and need to be brought back
up to a healthy voltage.

Signed-off-by: Fabian Pflug <fabian.pflug@grandcentrix.net>