power: battery: Fix parallel charger iio-channel names

Get IIO failure may occur due to parallel charger's iio-channel
names conflicting with main-charger/qg channels. Fix this by adding
"pl_" prefix for all parallel charger's iio-channels name.

Change-Id: I20cb8f984b38d7053890380ea024a2ef15cf5789
Signed-off-by: Ashish Chavan <ashichav@codeaurora.org>