iio: qcom-spmi-adc5: Add support for SW calibrated ADC

Add support for reading ADC channels on a PMIC7 ADC directly from
the ADC_PBS peripheral on that PMIC instead of through PMK8350
ADC peripheral using PBS, for targets where PMK8350 does not run in
master mode.

The conversion request sequence for using ADC_PBS is similar to
the existing one for ADC7, with some differences in register
writes. In addition, SW now performs calibration in place of PBS,
for every conversion, by reading the three calibration channels,
calculating the reference values and entering them in the
calibration registers.

Change-Id: I9e47ed69e75ee2580a37195ea9063f24ffb8aac5
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>