net: dsa: use ports list to setup multiple master devices

Now that we have a potential list of CPU ports, make use of it instead
of only configuring the master device of an unique CPU port.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>