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 Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

drivers/core/read.c

@@ -398,3 +398,8 @@ int dev_decode_display_timing(const struct udevice *dev, int index,
 {
 	return ofnode_decode_display_timing(dev_ofnode(dev), index, config);
 }
+
+ofnode dev_get_phy_node(const struct udevice *dev)
+{
+	return ofnode_get_phy_node(dev_ofnode(dev));
+}