For common DMA enguine, only slave_sg mode and cyclic mode is support.
However, SDMA can meet more kinds of DMA operation mode requirement. The
origin flags NORMAL and LOOP can no longer satisfy SDMA user's need.

In this patch,
	1 Construct a new enum sdma_mode to declare more kind of SDMA
	modes. This new variable would replace the old flags.
	2 Init sdma_mode to unvalid every time allocating a SDMA channel
	to avoid last SDMA channel configuration's impact.

Signed-off-by: Chen Liangjun <b36089@freescale.com>