power: qpnp-vm-bms: Add QPNP voltage mode(VM) BMS driver

The voltage-mode BMS driver uses periodic VBATT readings
to calculate the state of charge(SOC) of the battery.
The final SOC is calculated by an userspace application
after post-processing the VBATT readings.

Change-Id: I3c19e8dc7f1952c4e8274209d55aad176d90a180
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>