dmaengine: iop-adma: include prefetch.h

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

Link: https://lore.kernel.org/r/[e-mail]
Reported-by: kbuild test robot <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>
Acked-by: Vinod Koul <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>

drivers/dma/iop-adma.c

···
 #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>