net: dsa: document missing functions

Add description for the missing port_vlan_prepare, port_fdb_prepare,
port_fdb_dump functions in the DSA documentation.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>