Create a placeholder directory for each registered DMA device.

DMA drivers can use the dmaengine_get_debugfs_root() call to get their
debugfs root and can populate with custom files to aim debugging.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200306142839.17910-4-peter.ujfalusi@ti.comSigned-off-by: Vinod Koul <vkoul@kernel.org>