drivers/net/vsc9953: Add command for shared/private VLAN learning

drivers/net/vsc9953: Add command for shared/private VLAN learning

The command:
ethsw vlan fdb { [help] | show | shared | private }
 - make VLAN learning shared or private"

configures the FDB to share the FDB entries learned on multiple VLANs
or to keep them separated. By default, the FBD uses private VLAN
learning. This command has also been added to the ethsw generic parser
from common/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]>