dt-bindings: net: dsa: Let dsa.txt refer to dsa.yaml

The DSA bindings have been converted to YAML. Therefore, the old text style
documentation should refer to that one.

The text file can be removed completely once all the existing DSA switch
bindings have been converted as well.

Signed-off-by: Kurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>