dmaengine: pxa_dma: add debug information

Reuse the debugging features which were available in pxa architecture.
This is a copy of the code from arch/arm/plat-pxa/dma, which is doomed
to disappear once the conversion is completed towards dmaengine.

This is a transfer of the commit "[ARM] pxa/dma: add debugfs
entries" (d294948c

).

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>