hwmon: qpnp-current: Add configurable EOC option.

Provide a DT property qcom,iadc-poll-eoc option to
allow clients to choose between using interrupt or
polling method for end of conversion interrupt.
When the optional property is present, use polling
when an ADC conversion is requested.

Change-Id: I2c62a9f8e30f5c1200d439343f0548212977b731
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>