net: dsa: lantiq_gswip: prepare for more CPU port options

net: dsa: lantiq_gswip: prepare for more CPU port options

The MaxLinear GSW1xx series of switches support using either the
(R)(G)MII interface on port 5 or the SGMII interface on port 4 to be
used as CPU port. Prepare for supporting them by defining a mask of
allowed CPU ports instead of a single port.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/879c66672d26fe49c1f5d9aa40d8ebc0f31885ab.1755878232.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>