arm/dt: pm8941: Update vbat_sns channel property

Update the scaling property for vbat_sns to use
the correct reverse scaling function. Reverse scaling
functions are used to convert the input voltage to the
corresponding voltage threshold that is writen to the
VADC_TM threshold monitoring requests.

Change-Id: I1d5842a7b6fd4ff4be25658bc0e2ea3310a4e1f7
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>