hwmon: qpnp-adc: Update VADC property

Update VADC property to obtain the code for the full scale
voltage instead of the number of bits for the ADC. The
full scale code is more useful for scaling code to voltage.
Update the property as an optional property and use default
full scale code of 0x4000 for 1.875V.

Change-Id: I81b88ac2205f9e0753353230aa71c73711c378da
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>