Add binding document for the Dell Wyse 3020 a.k.a. "Ariel" Power Button.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201201083533.1724287-2-lkundrak@v3.sk


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>