dt-bindings: iio: adc: Add DT binding document for PMIC5 ADC

dt-bindings: iio: adc: Add DT binding document for PMIC5 ADC

PMIC5 ADC has support for clients to measure voltage and current
on inputs connected to the PMIC. Clients include reading voltage
phone power and on board system thermistors for thermal management.
ADC5 on certain PMIC has support to read battery current.

This change adds documentation.

Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>