dmaengine: dw: Enable runtime PM

When consumer requests channel power on the DMA controller device
and otherwise on the freeing channel resources.

Note, in some cases consumer acquires channel at the ->probe() stage and
releases it at the ->remove() stage. It will mean that DMA controller device
will be powered during all this time if there is no assist from hardware
to idle it. The above mentioned cases should be investigated separately
and individually.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Link: https://lore.kernel.org/r/20201103183938.64752-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>