net: dsa: Mock-up driver

This patch adds support for a DSA mock-up driver which essentially does
the following:

- registers/unregisters 4 fixed PHYs to the slave network devices
- uses eth0 (configurable) as the master netdev
- registers the switch as a fixed MDIO device against the fixed MDIO bus
  at address 31
- includes dynamic debug prints for dsa_switch_ops functions that can be
  enabled to get call traces

This is a good way to test modular builds as well as exercise the DSA
APIs without requiring access to real hardware. This does not test the
data-path, although this could be added later on.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/dsa_loop.h

@@ -0,0 +1,19 @@
+#ifndef __DSA_LOOP_H
+#define __DSA_LOOP_H
+
+struct dsa_chip_data;
+
+struct dsa_loop_pdata {
+	/* Must be first, such that dsa_register_switch() can access this
+	 * without gory pointer manipulations
+	 */
+	struct dsa_chip_data cd;
+	const char *name;
+	unsigned int enabled_ports;
+	const char *netdev;
+};
+
+#define DSA_LOOP_NUM_PORTS	6
+#define DSA_LOOP_CPU_PORT	(DSA_LOOP_NUM_PORTS - 1)
+
+#endif /* __DSA_LOOP_H */