net: dsa: allow drivers to get the port OF node

In the current DSA switch driver API, only the udevice of the switch
(belonging to UCLASS_DSA) is exposed, as well as an "int port" argument.
So drivers do not have access to the udevice of individual ports
(belonging to UCLASS_ETH), one of the reasons being that not all ports
have an associated UCLASS_ETH udevice.

However, all DSA ports have an OF node, and in some cases the driver
needs a handle to it, for all ports including the CPU port. Example: the
following Linux per-port device tree property:

	managed = "in-band-status";

states whether a port should operate with clause 37 in-band autoneg
enabled or not.

This patch exposes a function which can be called by individual drivers
as needed.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>

include/net/dsa.h

@@ -6,6 +6,7 @@
 #ifndef __DSA_H__
 #define __DSA_H__
 
+#include <dm/ofnode.h>
 #include <phy.h>
 #include <net.h>
 
@@ -145,6 +146,17 @@ int dsa_set_tagging(struct udevice *dev, ushort headroom, ushort tailroom);
  */
 struct udevice *dsa_get_master(struct udevice *dev);
 
+/**
+ * dsa_port_get_ofnode() - Return a reference to the given port's OF node
+ *
+ * Can be called at driver probe time or later.
+ *
+ * @dev:	DSA switch udevice pointer
+ * @port:	Port index
+ * @return OF node reference if OK, NULL on error
+ */
+ofnode dsa_port_get_ofnode(struct udevice *dev, int port);
+
 /**
  * dsa_port_get_pdata() - Helper that returns the platdata of an active
  *			(non-CPU) DSA port device.