hwmon: qpnp-adc-voltage: Add optional VADC property

Add option to select whether a fresh calibration
needs to be performed when performing the channel measurement.
If the property is not present the default value of
using calibration measurement collected from a timer
will be used. For most cases using a measurement collected
from a timer interval should suffice and the default value
can be used.

Change-Id: I632a9466f04d5d0712de436fc3e1251f7e2e7ad2
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>