iio: adc: Add QCOM SPMI PMIC5 ADC driver

This patch adds support for QCOM SPMI PMIC5 family
of ADC driver that supports hardware based offset and
gain compensation. The ADC peripheral can measure both
voltage and current channels whose input signal is
connected to the PMIC ADC AMUX.

The register set and configuration has been refreshed
compared to the prior QCOM PMIC ADC family. Register
ADC5 as part of the IIO framework.

Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>