power: battery: restore attribute name 'parallel_pct'

After converting to use ATTRIBUTE_GROUPS macro, attribute
'parallel_pct' is renamed to 'slave_pct'. Consequently,
Thermal Balancer in userspace quits and stops working.

Change it back.

Change-Id: I3199754b68e469bbd5c0951e8dd080b732349b86
Signed-off-by: Harry Yang <harryy@codeaurora.org>