arm/dt: pm8941: Add additional VADC channel nodes

Add additional main-mux voltage channels to read ADC
from VADC USRP peripheral. The new channels have different
pre-div scaling compared to previous channel voltage node.
The new channel nodes include support to read vbatt, vcharger
that can be used by the battery driver. In addition, the
625mV and 1.125V channel nodes are added to verify the
reference channels used for absolute calibration.

Change-Id: I53265a00b95ba4f5fb726731532070b8b758d86a
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>