net: ocelot: Add PGID_BLACKHOLE

Add a new PGID that is used not to forward frames anywhere. It is used
by MRP to make sure that MRP Test frames will not reach CPU port.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>