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/ofnode.c

@@ -1198,3 +1198,24 @@ const char *ofnode_conf_read_str(const char *prop_name)
 
 	return ofnode_read_string(node, prop_name);
 }
+
+ofnode ofnode_get_phy_node(ofnode node)
+{
+	/* DT node properties that reference a PHY node */
+	static const char * const phy_handle_str[] = {
+		"phy-handle", "phy", "phy-device",
+	};
+	struct ofnode_phandle_args args = {
+		.node = ofnode_null()
+	};
+	int i;
+
+	assert(ofnode_valid(node));
+
+	for (i = 0; i < ARRAY_SIZE(phy_handle_str); i++)
+		if (!ofnode_parse_phandle_with_args(node, phy_handle_str[i],
+						    NULL, 0, 0, &args))
+			break;
+
+	return args.node;
+}