hwmon: qpnp-voltage: Support multiple device instance

Support multiple voltage ADC device instance. Clients
need to register with the voltage ADC using the device
consumer name used to associate with the corresponding
phandle of the VADC. Clients receive the device instance
which is to be used everytime an ADC request is made.

This change involves changing the interface to issue
ADC request and the corresponding clients changes.

Change-Id: If0557bc657fecd172a8cf960900ee93b4bce37f8
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>