mxs-dma : move the mxs dma.h to a more common place

Move the header to a more common place.
The mxs dma engine is not only used in mx23/mx28, but also used
in mx50/mx6q.  It will also be used in the future chips.

Rename it to mxs-dma.h, and create a new folder include/linux/fsl/ to
store the Freescale's header files.

change mxs-dma driver, mxs-mmc driver, gpmi-nand driver, mxs-saif driver
to the new header file.

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Huang Shijie <b32955@freescale.com>
Acked-by: Vinod Koul <vinod.koul@linux.intel.com>
Acked-by: Chris Ball <cjb@laptop.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>