dt-bindings:iio:adc:nuvoton,nau7802 yaml conversion

A simple conversion from txt file to yaml.  I added the #io-channel-cells
property as optional to allow the channels of this ADCs to be used
to provide services to other drivers, for example if an analog
accelerometer is connected.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>
Cc: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20200909175946.395313-8-jic23@kernel.org