drivers/net/vsc9953: Add commands to manipulate the FDB for VSC9953

The new command:
ethsw [port <port_no>] [vlan <vid>] fdb
        { [help] | show | flush | { add | del } <mac> }

Can be used to add and delete FDB entries. Also, the command can be used
to show entries from the FDB tables. When used with [port <port_no>]
and [vlan <vid>], only the matching the FDB entries can be seen or
flushed. The command has also been added to the generic ethsw parser
from 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>