Added Serial USART peripheral driver for the XMEGA platform.

Fix XMEGA SPI peripheral driver's incorrect const-ness on the SPI peripheral struct.

LUFA/Drivers/Peripheral/AVR8/Serial_AVR8.c

@@ -82,7 +82,8 @@ void Serial_SendString(const char* StringPtr)
 	}
 }
 
-void Serial_SendData(const uint8_t* Buffer, uint16_t Length)
+void Serial_SendData(const uint8_t* Buffer,
+                     uint16_t Length)
 {
 	while (Length--)
 	  Serial_SendByte(*(Buffer++));