dw_dmac: implement a private tx_list

Drop dw_dmac's use of tx_list from struct dma_async_tx_descriptor in
preparation for removal of this field.

Cc: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --cc drivers/dma/dw_dmac.c
Simple merge

diff --cc drivers/dma/dw_dmac_regs.h
Simple merge