Revert "SPI library to new format"

(Filtered from arduino/Arduino@d5c8287)

Author: cmaglie

arch/avr/SPI.cpp renamed to SPI.cpp

@@ -9,7 +9,7 @@
  */
 
 #include "pins_arduino.h"
-#include "SPI_Class.h"
+#include "SPI.h"
 
 SPIClass SPI;