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

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 <[email protected]>
Signed-off-by: Codrin Ciubotariu <[email protected]>
Acked-by: Joe Hershberger <[email protected]>
Reviewed-by: York Sun <[email protected]>