power: qpnp-fg-gen4: Add GEN4 FG driver

Add support for GEN4 Fuel Gauge (FG) peripheral that is present
on PM855B. FG measures and provides various battery parameters
like Voltage, Current, State of Charge, Temperature to the
clients upon request.

Change-Id: Icd2bac7fb38e7d4e54f10f89978c263b450aedf8
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>