power: pm8921-charger: resume charging based on SOC

At EOC, the battery is disconnected from the charger and starts
discharging. This can result in SOC dropping to around 93% before
charging starts again.

Manually set the CHG_VBAT_DET threshold high to restart charging once SOC
drops below keep_charge_percent.

CRs-Fixed: 404909
Change-Id: I07577d1aa57bfa4bee40b52c18b5bfcb1cdb7070
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>