Add RTL8201F PHY support, make PHY_TYPE overridable

Add RTL8201F as a third PHY option and wrap PHY_TYPE in an ifndef guard so it can be overridden at build time without modifying the header.

Author: vanvught

lib-gd32/gd32f20x/GD32F20x_standard_peripheral/Include/gd32f20x_enet.h

@@ -62,9 +62,12 @@ OF SUCH DAMAGE.
 /* #define USE_DELAY */
 
 #ifndef _PHY_H_
-#define DP83848                          0U
-#define LAN8700                          1U
-#define PHY_TYPE                         DP83848
+#define DP83848 0
+#define LAN8700 1
+#define RTL8201F 2       // AvV
+#ifndef PHY_TYPE         // AvV
+#define PHY_TYPE DP83848 // AvV
+#endif                   // AvV
 
 #define PHY_ADDRESS                      ((uint16_t)1U)                         /*!< phy address determined by the hardware */