dt-bindings: iio/adc: Add bindings for Ingenic JZ47xx SoCs ADC.

Add device tree bindings for the ADC controller on JZ47xx SoCs,
used by the ingenic-adc driver.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>