phy: Fix warning due to missing definition of structure

Fix this sort of warning if generic-phy.h is included:

include/generic-phy.h:52:42: warning: ‘struct ofnode_phandle_args’ declared inside parameter list will not be visible outside of this definition or declaration
  int (*of_xlate)(struct phy *phy, struct ofnode_phandle_args *args);

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>

include/generic-phy.h

@@ -7,6 +7,7 @@
 #ifndef __GENERIC_PHY_H
 #define __GENERIC_PHY_H
 
+struct ofnode_phandle_args;
 
 /**
  * struct phy - A handle to (allowing control of) a single phy port.