hwmon: qpnp-adc-voltage: Add interval mode select option

Enable interval mode before starting measurement interval
request and disable interval mode during single conversion
request. Add additional checks for high and low threshold
interrupt routine to check the mode is in recurring
measurement mode before threshold notification.

Change-Id: I8d1a92c801d2a8ad7b61305c146718aecaca9888
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>