hwmon: qpnp-vadc: Add EOC polling option

Add a DT property qcom,vadc-poll-eoc option to allow
clients to use either interrupt mode or polling mode
for end of conversion request. The DT property is a
optional property to specify polling. If this property is not
present default to use interrupt mode.

Change-Id: I16f3677a24200b80dc5a8535a98ec4a44158302c
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>