ARM: dts: orion5: 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 Orion5x Ethernet is an RGMII port. Set the
switch to impose the RGMII 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>