dsa: mv88x6xxx: Add debugfs interface for device map

The device map is used to route packets between cascaded switches.
Add dumping a switches device map via debugfs.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/dsa/mv88e6xxx.h b/drivers/net/dsa/mv88e6xxx.h
index 7cccff2..a2c9ac0 100644
--- a/drivers/net/dsa/mv88e6xxx.h
+++ b/drivers/net/dsa/mv88e6xxx.h
@@ -260,6 +260,7 @@
 #define GLOBAL2_DEVICE_MAPPING	0x06
 #define GLOBAL2_DEVICE_MAPPING_UPDATE		BIT(15)
 #define GLOBAL2_DEVICE_MAPPING_TARGET_SHIFT	8
+#define GLOBAL2_DEVICE_MAPPING_PORT_MASK	0x0f
 #define GLOBAL2_TRUNK_MASK	0x07
 #define GLOBAL2_TRUNK_MASK_UPDATE		BIT(15)
 #define GLOBAL2_TRUNK_MASK_NUM_SHIFT		12