PHY status API for ethernet drivers (#8784)

* PHY status API for W5500 & ENC28J60 drivers
* move linkStatus() from ArduinoEthernet:: to LwipIntfDev::
* LwipIntfDev: include PHY status into ::connected()

libraries/lwIP_w5500/src/utility/w5500.h

@@ -79,6 +79,24 @@ public:
     */
     uint16_t readFrame(uint8_t* buffer, uint16_t bufsize);
 
+    /**
+        Check physical link
+        @return true when physical link is up
+    */
+    bool isLinked()
+    {
+        return wizphy_getphylink() == PHY_LINK_ON;
+    }
+
+    /**
+        Report whether ::isLinked() API is implemented
+        @return true when ::isLinked() API is implemented
+    */
+    constexpr bool isLinkDetectable() const
+    {
+        return true;
+    }
+
 protected:
     static constexpr bool interruptIsPossible()
     {