dt-bindings: iio: adc: add binding for X-Powers AXP PMICs ADC

X-Powers PMICs have several ADC channels that can be used for different
purposes, e.g. PMIC internal temperature, battery voltage or AC current.

This is the documentation for AXP209, AXP221/223 and AXP813 ADC
bindings.

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>