Debugging options for the DMA engine subsystem

This adds Kconfig options for DEBUG and VERBOSE_DEBUG to the DMA
engine subsystem, I got tired of editing the Makefile manually
each time I want to debug things in here, modelled this on the
debug switches for other subsystems and works like a charm when
working on our DMA engines.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>