net: dsa: introduce the dsa_cpu_ports() helper

Similar to dsa_user_ports() which retrieves a port mask of all user
ports, introduce dsa_cpu_ports() which retrieves the mask of all CPU
ports of a switch.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>