dmaengine: imx-dma: cleanup scatterlist layering violations

This dma engine driver directly accesses page_link assuming knowledge
that should be contained only in scatterlist.h.

We replace these with calls to sg_chain and sg_assign_page.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Stephen Bates <sbates@raithlin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Per Förlin <per.forlin@axis.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>