ARM: dts: vf610: ZII: 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 Vybrid FEC is a Fast Ethrnet using RMII.

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

Lastly, for DSA links between switches, add a fixed-link node
indicating the expected speed/duplex of the link.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>