arm/dt: pm8941: Modify VADC channel property nodes

Update the channel properties for channels - usb_in,
vchg_sns, vcoin, vbat_sns, vph_pwr and reference
voltage channels 0.625V and 1.25V. The changes for
the channel usb_in, vchg_sns, vcoin, vbat_sns, vph_pwr
ref_625mv and ref_1250v is to update the pre-div scaling
ratio of 1/20, 1/6, 1/3, 1/3, 1/3, 1, 1 respectively.
Also update the type for ref_1250v channel node. The
reference channels are .625mv and 1.250V. Previously
the ref_1250v was incorrectly referenced as ref_1125v.

Change-Id: I91bee89229b54ba41ca118713a462df7bb9f55a5
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>