net: dsa: add ports list in the switch fabric

Add a list of switch ports within the switch fabric. This will help the
lookup of a port inside the whole fabric, and it is the first step
towards supporting multiple CPU ports, before deprecating the usage of
the unique dst->cpu_dp pointer.

In preparation for a future allocation of the dsa_port structures,
return -ENOMEM in case no structure is returned, even though this
error cannot be reached yet.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>