net: dsa: define port types

Introduce an enumerated type for ports, which will be way more explicit
to identify a port type instead of digging into switch port masks.

A port can be of type CPU, DSA, user, or unused by default. This is a
static parsed information that cannot be changed at runtime.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>