dma: move shdma driver to an own directory

The shdma driver is going to be split into multiple files. To make this more
convenient move it to an own directory.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>