hwmon: qpnp-adc-voltage: Add measurement interval mode

Support clients to issue measurement interval mode on a given
channel. VADC_USR can only support one measurement interval
mode request. When any client issues a single VADC request,
the channel on which measurement interval mode is being
requested will be put on hold, the single ADC conversion
request handled and the channel for the measurement interval
will resume. The measurement interval mode request allows
clients to set thresholds and recieve notification once a
threshold is crossed.

Change-Id: If99f4ab43ec1cd9b838ace7fb358bf3e426e48fd
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>