dt-bindings: dsa: sync with maintainers

The MAINTAINERS file has Andrew Lunn, Florian Fainelli, and Vladimir Oltean
listed as the maintainers for generic dsa bindings. Update dsa.yaml and
dsa-port.yaml accordingly.

Signed-off-by: Colin Foster <colin.foster@in-advantage.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>