net: dsa: add one port parsing function per type

Add dsa_port_parse_user, dsa_port_parse_dsa and dsa_port_parse_cpu
functions to factorize the code shared by both OF and pdata parsing.

They don't do much for the moment but will be extended later to support
tagging protocol resolution for example.

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