dmaengine/ste_dma40: support pm in dma40

This patch adds power management support to the dma40
driver. The DMA registers are backed up and restored,
during suspend/resume. Also flags to track the dma usage
have been introduced to facilitate this. Patch also includes
few other minor changes, related to formatting, comments.

Signed-off-by: Narayanan G <narayanan.gopalakrishnan@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>