dsa: mv88e6xxx: Allow speed/duplex of port to be configured

The current code sets user ports to perform auto negotiation using the
phy. CPU and DSA ports are configured to full duplex and maximum speed
the switch supports.

There are however use cases where the CPU has a slower port, and when
user ports have SFP modules with fixed speed. In these cases, port
settings to be read from a fixed_phy devices. The switch driver then
needs to implement the adjust_link op, so the port settings can be
set.

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>