drivers/net/vsc9953: Add VLAN commands for VSC9953

The new added commands can be used to configure VLANs for a port
on both ingress and egress.

The new commands are:
ethsw [port <port_no>] pvid { [help] | show | <pvid> }
 - set/show PVID (ingress and egress VLAN tagging) for a port;
ethsw [port <port_no>] vlan { [help] | show | add <vid> | del <vid> }
 - add a VLAN to a port (VLAN members);
ethsw [port <port_no>] untagged { [help] | show | all | none | pvid }
 - set egress tagging mod for a port"
ethsw [port <port_no>] egress tag { [help] | show | pvid | classified }
 - Configure VID source for egress tag. Tag's VID could be the
   frame's classified VID or the PVID of the port
These commands have also been added to the ethsw generic parser from
common/cmd_ethsw.c

Signed-off-by: Johnson Leung <johnson.leung@freescale.com>
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@freescale.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: York Sun <yorksun@freescale.com>