Documentation: devicetree: update dsa binding with optional properties

Add documentation for a bunch of new optional properties described in
ethernet.txt and fixed-link.txt, this includes: 'phy-handle', 'phy-mode'
and the 'fixed-link' subnode.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>