dma: shdma: prepare for conversion to the shdma base library

By placing an anonymous union at the top of struct sh_dmae_slave we can
transparently prepare all device and client drivers for the upcoming
shdma-base conversion.

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