dt-bindings:iio:adc:st,stmpe-adc yaml conversion

Conversion from freeform text to yaml.
One oddity in this binding is that, for historical reasons it requires
the node name to be stmpe_adc.  I've put that in the decription field
but I'm not sure if there is a better way to specify this?

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Cc: Stefan Agner <stefan@agner.ch>
Link: https://lore.kernel.org/r/20200909175946.395313-4-jic23@kernel.org