[MTD] XIP: Use generic xip_iprefetch() instead of asm volatile (...)

Untested, but shouldn't break anything... Makes MTD_XIP arch
independent. I guess this is why xip_iprefetch() was made for.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>