dmaengine: dw-edma: Use DMA engine device debugfs subdirectory

Since all DW eDMA read and write channels are now installed in a framework
of a single DMA engine device, move all the DW eDMA-specific debugfs nodes
into a ready-to-use DMA-engine debugfs subdirectory. It's created during
the DMA-device registration and can be found in the dma_device.dbg_dev_root
field.

Link: https://lore.kernel.org/r/20230113171409.30470-19-Sergey.Semin@baikalelectronics.ru


Tested-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Vinod Koul <vkoul@kernel.org>