dmaengine: edma: Document variables used for residue accounting

The granular residue accounting code uses certain variables specifically
for residue accounting. Document these in the structure declaration.
Also move around some elements and group them together.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>