The Trunk Mask and Trunk Mapping registers are two Global 2 indirect
accesses to trunking configuration.

Add helpers for these tables and simplify the Global 2 setup.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>