power: qpnp-bms: Notify BMS of charging status

BMS relies on charger for notifications about battery presence and
charging status. Instead of using private APIs or setting power
supply properties, simply register BMS as a supplicant of battery
so that when charger calls power supply changed on battery, BMS
gets notified about charging status.

Change-Id: Ifa4ac94ea825f2b0a1149ded4880f1088af5832b
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>