of: add of_mdio_find_device() api

Add a helper function which finds the mdio_device structure given a
device tree node. This is helpful for finding the PCS device based on a
DTS node but managing it as a mdio_device instead of a phy_device.

Signed-off-by: Russell King <[e-mail]>
Signed-off-by: Ioana Ciornei <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/of/of_mdio.c

···
 EXPORT_SYMBOL(of_mdiobus_register);
 
 /**
+ * of_mdio_find_device - Given a device tree node, find the mdio_device
+ * @np: pointer to the mdio_device's device tree node
+ *
+ * If successful, returns a pointer to the mdio_device with the embedded
+ * struct device refcount incremented by one, or NULL on failure.
+ * The caller should call put_device() on the mdio_device after its use
+ */
+struct mdio_device *of_mdio_find_device(struct device_node *np)
+{
+	struct device *d;
+
+	if (!np)
+		return NULL;
+
+	d = bus_find_device_by_of_node(&mdio_bus_type, np);
+	if (!d)
+		return NULL;
+
+	return to_mdio_device(d);
+}
+EXPORT_SYMBOL(of_mdio_find_device);
+
+/**
  * of_phy_find_device - Give a PHY node, find the phy_device
  * @phy_np: Pointer to the phy's device tree node
  *
···
  */
 struct phy_device *of_phy_find_device(struct device_node *phy_np)
 {
-	struct device *d;
 	struct mdio_device *mdiodev;
 
-	if (!phy_np)
+	mdiodev = of_mdio_find_device(phy_np);
+	if (!mdiodev)
 		return NULL;
 
-	d = bus_find_device_by_of_node(&mdio_bus_type, phy_np);
-	if (d) {
-		mdiodev = to_mdio_device(d);
-		if (mdiodev->flags & MDIO_DEVICE_FLAG_PHY)
-			return to_phy_device(d);
-		put_device(d);
-	}
+	if (mdiodev->flags & MDIO_DEVICE_FLAG_PHY)
+		return to_phy_device(&mdiodev->dev);
+
+	put_device(&mdiodev->dev);
 
 	return NULL;
 }

include/linux/of_mdio.h

···
 int of_mdiobus_register(struct mii_bus *mdio, struct device_node *np);
 int devm_of_mdiobus_register(struct device *dev, struct mii_bus *mdio,
 			     struct device_node *np);
+struct mdio_device *of_mdio_find_device(struct device_node *np);
 struct phy_device *of_phy_find_device(struct device_node *phy_np);
 struct phy_device *
 of_phy_connect(struct net_device *dev, struct device_node *phy_np,
···
 	 */
 
 	return mdiobus_register(mdio);
+}
+
+static inline struct mdio_device *of_mdio_find_device(struct device_node *np)
+{
+	return NULL;
 }
 
 static inline struct phy_device *of_phy_find_device(struct device_node *phy_np)