net: dsa: move fixed link registration helpers

The new bindings (dsa2.c) and the old bindings (legacy.c) share two
helpers dsa_cpu_dsa_setup and dsa_cpu_dsa_destroy, used to register or
deregister a fixed PHY if a given port has a corresponding device node.

Unclutter the code by moving them into two new port.c helpers,
dsa_port_fixed_link_register_of and dsa_port_fixed_link_(un)register_of.

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