dt-bindings: net: dsa: Add documentation for Hellcreek switches

Add basic documentation and example.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>