net: dsa: Split up creating/destroying of DSA and CPU ports

Refactor the code to setup a single DSA/CPU port into a function of
its own, and export it, so it can be used by the new binding.

Similarly, refactor the destroy code into a function.  When destroying
the ports, don't put the of node. They should be released at the end
along with the normal ports.

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