dt-bindings:iio:adc:motorola,cpcap-adc yaml conversion

Conversion from txt to yaml.
Slightly expanded example to give a bit more context.
Description lifted from the original driver commit.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20200909175946.395313-12-jic23@kernel.org