net: dsa: Allow DSA and CPU ports to have a phy-mode property

It can be useful for DSA and CPU ports to have a phy-mode property, in
particular to specify RGMII delays. Parse the property and set it in
the fixed-link phydev.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>