iio: iadc: Qualcomm SPMI PMIC current ADC driver

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

The driver registers itself through IIO interface.

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