drivers/net/vsc9953: Add command to show/clear port counters

The new added command:
ethsw [port <port_no>] statistics { [help] | [clear] }

will print counters like the number of Rx/Tx frames,
number of Rx/Tx bytes, number of Rx/Tx unicast frames, etc.
This patch also adds this commnd in the genereric ethsw
parser from cmd_ethsw.c

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