hwmon: qpnp-current: Support multiple instance

Support multiple instance of the IADC device. Clients
need to register with the current ADC using the device
consumer name used to associate with the corresponding
phandle of the IADC. 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: I51423395ed7282669ca7fbc7dc585e0292417e71
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>