dmaengine: iop-adma: include prefetch.h

Compile-testing this driver fails on m68k without the
extra header inclusion.

Link: https://lore.kernel.org/r/20190809163334.489360-2-arnd@arndb.de
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

drivers/dma/iop-adma.c

@@ -16,6 +16,7 @@
 #include <linux/spinlock.h>
 #include <linux/interrupt.h>
 #include <linux/platform_device.h>
+#include <linux/prefetch.h>
 #include <linux/memory.h>
 #include <linux/ioport.h>
 #include <linux/raid/pq.h>