power: qpnp-fg: use real time clock to track SRAM updates

Currently, if a SRAM property is read by an external driver, the fuel
gauge driver schedules an SRAM update 1 second after the read. If a
property is continuously read, the SRAM could never be updated because
the SRAM update is continuously pushed backed 1 second.

Fix this by tracking the time of the last SRAM data update using the RTC
and always doing the SRAM update if the last read is stale enough.

Change-Id: I265746fa4715c6d7476c6c531fb82db64100ba6e
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>