net: dsa: add port parse functions

Add symmetrical DSA port parsing functions for pdata and device tree,
used to parse and validate a given port node or platform data.

They don't do much for the moment but will be extended later on to
assign a port type and get device references.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>