ARM: dts: armada: Add missing phy-mode and fixed links

The DSA framework has got more picky about always having a phy-mode
for the CPU port. The Armada Ethernet supports RGMII, SGMII,
1000base-x and 2500Base-X. Set the switch phy-mode based on how the
SoC Ethernet ports is been configured. For RGMII mode, have the switch
add the delays.

Additionally, the cpu label has never actually been used in the
binding, so remove it.

Lastly, add a fixed-link node indicating the expected speed/duplex of
the link to the SoC.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>