dmaengine: ioatdma: add descriptor pre-fetch support for v3.4

Adding support for new feature on ioatdma 3.4 hardware that provides
descriptor pre-fetching in order to reduce small DMA latencies.

Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>