iio: vadc: Qualcomm SPMI PMIC voltage ADC driver

The voltage ADC is peripheral of Qualcomm SPMI PMIC chips. It has
15bits resolution and register space inside PMIC accessible across
SPMI bus.

The vadc driver registers itself through IIO interface.

Signed-off-by: Stanimir Varbanov <svarbanov@mm-sol.com>
Signed-off-by: Ivan T. Ivanov <iivanov@mm-sol.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>