dt-bindings: net: dsa: Add DSA yaml binding

For future DSA drivers it makes sense to add a generic DSA yaml binding which
can be used then. This was created using the properties from dsa.txt. It
includes the ports and the dsa,member property.

Suggested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>