net: dsa: Allow configuration of CPU & DSA port speeds/duplex

By default, DSA and CPU ports are configured to the maximum speed the
switch supports. However there can be use cases where the peer devices
port is slower. Allow a fixed-link property to be used with the DSA
and CPU port in the device tree, and use this information to configure
the port.

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