dt-bindings:iio:adc:ti,palmas-gpadc: txt to yaml format conversion.

Simple conversion of the binding doc for this subnode of the palmas
PMIC.
Given age of driver and lack interaction with original authors,
I've guessed at Tony for a maintainer on this one.  Tony, if you
are happy with that great, otherwise I can default back to myself.

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/20201031184854.745828-35-jic23@kernel.org