Add struct mip_spi for SPI drivers, and skeleton for enc28j60 driver

mongoose.h

@@ -1426,9 +1426,9 @@ struct mip_driver {
   void (*init)(uint8_t *mac, void *data);           // Initialise driver
   size_t (*tx)(const void *, size_t, void *data);   // Transmit frame
   size_t (*rx)(void *buf, size_t len, void *data);  // Receive frame (polling)
-  bool (*status)(void *data);                       // Up/down status
+  bool (*up)(void *data);                           // Up/down status
   // Set receive callback for interrupt-driven drivers
-  void (*rxcb)(void (*fn)(void *buf, size_t len, void *rxdata), void *rxdata);
+  void (*setrx)(void (*fn)(void *buf, size_t len, void *rxdata), void *rxdata);
 };
 
 struct mip_ipcfg {
@@ -1439,6 +1439,15 @@ struct mip_ipcfg {
 void mip_init(struct mg_mgr *, struct mip_ipcfg *, struct mip_driver *, void *);
 
 extern struct mip_driver mip_driver_stm32;
+extern struct mip_driver mip_driver_enc28j60;
+
+// Drivers that require SPI, can use this SPI abstraction
+struct mip_spi {
+  void *spi;                        // Opaque SPI bus descriptor
+  uint8_t (*txn)(void *, uint8_t);  // SPI transaction: write 1 byte, read reply
+  void (*begin)(void *);            // SPI begin: slave select low
+  void (*end)(void *);              // SPI end: slave select high
+};
 
 #ifdef __cplusplus
 }