hwmon: qpnp-adc: Use buffered 625mV reference channel

Use channel 0xc to read the buffered 625mV reference
channel during calibration if it exists. The channel
may not exist on all the PMIC versions. If the channel
is supported in the PMIC add it as a channel in the
VADC DT channel node.

Change-Id: I49b54e161538f075fcd89ed9beb7ffbdd8034a87
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>