dt-bindings: net: dsa: b53: Add YAML bindings

Convert the b53 DSA device tree bindings to YAML in order to allow
for automatic checking and such.

Reviewed-by: Rob Herring <robh@kernel.org>
Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kurt Kanzenbach <kurt@kmk-computers.de>