power: pm8921-bms: use configurable poll times

The current code calculates the state of charge every 20 seconds.
We need to increase the frequency when the voltage drops to near
cutoff values.

Make the poll interval configurable, choose 1 second interval when
the battery nears the cutoff voltage and 20 seconds otherwise.

Change-Id: I1bc779e5890f6994442c562ee5abf37c06dc6d3d
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>