Add enc28j60 driver skeleton

mip/driver_enc28j60.c

@@ -0,0 +1,27 @@
+#include "mip.h"
+
+#if MG_ENABLE_MIP
+static void mip_driver_enc28j60_init(uint8_t *mac, void *data) {
+  (void) mac, (void) data;
+}
+
+static size_t mip_driver_enc28j60_tx(const void *buf, size_t len, void *data) {
+  (void) buf, (void) len, (void) data;
+  return 0;
+}
+
+static size_t mip_driver_enc28j60_rx(void *buf, size_t len, void *data) {
+  (void) buf, (void) len, (void) data;
+  return 0;
+}
+
+static bool mip_driver_enc28j60_up(void *data) {
+  (void) data;
+  return false;
+}
+
+struct mip_driver mip_driver_enc28j60 = {.init = mip_driver_enc28j60_init,
+                                         .tx = mip_driver_enc28j60_tx,
+                                         .rx = mip_driver_enc28j60_rx,
+                                         .up = mip_driver_enc28j60_up};
+#endif