dt-bindings: net: dsa: Document sfp and managed properties

The 'sfp' and 'managed' properties are commonly used to describe
Ethernet switch ports connecting to SFP/SFF cages, describe these two
properties as valid that we inherit from ethernet-controller.yaml.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>