dmaengine: fsl-edma: fixup reg offset and hw S/G support in big-endian model

The offset of all 8-/16-bit registers in big-endian eDMA model are
swapped in a 32-bit size opposite those in the little-endian model.

The hardware Scatter/Gather requires the subsequent TCDs stored in memory
in little endian independent of the register endian model, the eDMA engine
will do the swap if need.

This patch also use regular assignment for tcd variables r/w
instead of with io function previously that may not always be true.

Signed-off-by: Jingchang Lu <jingchang.lu@freescale.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>