power: qpnp-charger: use iadc for ibat trim logic

The CURRENT_NOW property provided through BMS
only provides averaged ibat values. This is not
ideal for the ibat trim logic which requires
instantenous current readings.

Fix this by using the iadc peripheral to read
the INTERNAL_RSENSE or EXTERNAL_RSENSE channel.

Change-Id: I654f8b0d17e081bd7bc572165ac619a7d412bdaf
Signed-off-by: David Keitel <dkeitel@codeaurora.org>