dt-bindings:iio:adc:x-powers,axp209-adc: txt to yaml conversion

This is a very small binding.  It might make sense at some stage
to just roll it into the parent mfd.  For now, converted as is.
The main advantage of this document is the identification of the
channel index values when this is used as a provider of ADC channels
to consumers.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201031184854.745828-32-jic23@kernel.org