dma: ste_dma40: Fixup system suspend/resume

Make sure to handle register context save/restore when needed from
system PM callbacks.

Previously we solely trusted the device to reside in in-active state
while the system suspend callback were invoked, which is just too
optimistic.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>