net: dsa: mv88e6xxx: implement port_link_state for mv88e6250

The mv88e6250 has a rather different way of reporting the link, speed
and duplex status. A simple difference is that the link bit is bit 12
rather than bit 11 of the port status register.

It gets more complicated for speed and duplex, which do not have
separate fields. Instead, there's a four-bit PortMode field, and
decoding that depends on whether it's a phy or mii port. For the phy
ports, only four of the 16 values have defined meaning; the rest are
called "reserved", so returning {SPEED,DUPLEX}_UNKNOWN seems
reasonable.

For the mii ports, most possible values are documented (0x3 and 0x5
are reserved), but I'm unable to make sense of them all. Since the
bits simply reflect the Px_MODE[3:0] configuration pins, just support
the subset that I'm certain about. Support for other setups can be
added later.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>