dt-bindings: net: dsa: Document dsa-tag-protocol property

The 'dsa-tag-protocol' is used to force a switch tree to use a
particular tag protocol, typically because the Ethernet controller
that it is connected to is not compatible with the default one.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Signed-off-by: David S. Miller <davem@davemloft.net>