power: fg-util: Update getting battery voltage and current from FG

Currently, battery voltage and current are obtained from FG by
reading VBATT and IBATT registers in FG_BATT_INFO peripheral. To
get a more reliable reading, read the shadow registers VBATT_CP
and IBATT_CP as well. If they match, then use them to show it to
the clients.

Change-Id: Idb5d57e3dd9b9c2a33e77c1b874823fb087c54d9
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>