dmaengine: mxs: Add header file to be shared with gpmi nand driver

The mxs dma driver can do PIO transfers. A pointer to the PIO words
to transfer is passed in the struct scatterlist * argument of
dmaengine_prep_slave_sg(). It's quite ugly and non obvious to cast
u32 * to struct scatterlist * each time when calling
dmaengine_prep_slave_sg(), so add a static inline wrapper function
to be called by the user along with a description what is going on.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>