usb: musb: blackfin: include prefetch head file

After the prefetch/list.h restructure, drivers need to explicitly include
linux/prefetch.h in order to use the prefetch() function.  Otherwise, the
current driver fails to build:
drivers/usb/musb/blackfin.c: In function 'musb_write_fifo':
drivers/usb/musb/blackfin.c:43: error: implicit declaration of function
'prefetch'
Signed-off-by: NBob Liu <lliubbo@gmail.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/musb/blackfin.c

@@ -17,6 +17,7 @@
 #include <linux/io.h>
 #include <linux/platform_device.h>
 #include <linux/dma-mapping.h>
+#include <linux/prefetch.h>
 
 #include <asm/cacheflush.h>