net: introduce helpers to get PHY ofnode from MAC

Add helpers ofnode_get_phy_node() and dev_get_phy_node() and use it in
net/mdio-uclass.c function dm_eth_connect_phy_handle(). Also add
corresponding UT test.

This is useful because other part's of U-Boot may want to get PHY ofnode
without connecting a PHY.

Signed-off-by: Marek BehĂșn <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/read.h b/include/dm/read.h
index 233af3c..899eb81 100644
--- a/include/dm/read.h
+++ b/include/dm/read.h
@@ -743,6 +743,20 @@
 int dev_decode_display_timing(const struct udevice *dev, int index,
 			      struct display_timing *config);
 
+/**
+ * dev_get_phy_node() - Get PHY node for a MAC (if not fixed-link)
+ *
+ * This function parses PHY handle from the Ethernet controller's ofnode
+ * (trying all possible PHY handle property names), and returns the PHY ofnode.
+ *
+ * Before this is used, ofnode_phy_is_fixed_link() should be checked first, and
+ * if the result to that is true, this function should not be called.
+ *
+ * @dev: device representing the MAC
+ * Return: ofnode of the PHY, if it exists, otherwise an invalid ofnode
+ */
+ofnode dev_get_phy_node(const struct udevice *dev);
+
 #else /* CONFIG_DM_DEV_READ_INLINE is enabled */
 #include <asm/global_data.h>
 
@@ -1092,6 +1106,11 @@
 	return ofnode_decode_display_timing(dev_ofnode(dev), index, config);
 }
 
+static inline ofnode dev_get_phy_node(const struct udevice *dev)
+{
+	return ofnode_get_phy_node(dev_ofnode(dev));
+}
+
 #endif /* CONFIG_DM_DEV_READ_INLINE */
 
 /**