power: qpnp-smbcharger: support reading voltage

Currently, POWER_SUPPLY_VOLTAGE_NOW is only exported by the bms
power supply because it is provided by the fuel gauge peripheral.
However, android looks for voltage under /sys/class/power_supply/
battery, which currently does not support reading the voltage.

Fix this by exporting this property in the battery power supply as
well, by making them read through to the bms power supply.

CRs-Fixed: 725746
Change-Id: I1126bec4f4eba7cbc4227027dc452331d85841f3
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>