power: supply: Add pm8916 VM-BMS support

This driver adds basic support for VM-BMS found in pm8916.

VM-BMS is a very basic fuel-gauge hardware block that is, sadly,
incapable of any gauging. The hardware supports measuring OCV in
sleep mode, where the battery is not in use, or measuring average
voltage over time when the device is active.

This driver implements basic value readout from this block.

Signed-off-by: Nikita Travkin <nikita@trvn.ru>
Link: https://lore.kernel.org/r/20230915-pm8916-bms-lbc-v3-3-f30881e951a0@trvn.ru
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>