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>