dt-bindings: iio/adc: Convert ingenic-adc docs to YAML.

Convert the textual documentation of Device Tree bindings for the
Ingenic JZ47xx SoCs ADC controller to YAML.

The `interrupts` property is now explicitly listed and marked as
required. While missing from the previous textual documentation, this
property has been used with all the boards which probe this driver.

Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Tested-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>