dma: edma: Split out PaRAM set calculations into its own function

PaRAM set calculation is abstracted into its own function to
enable better reuse for other DMA cases such as cyclic. We adapt
the Slave SG case to use the new function.

This provides a much cleaner abstraction to the internals of the
PaRAM set. However, any PaRAM attributes that are not common to
all DMA types must be set separately such as setting of interrupts.
This function takes care of the most-common attributes.

Also added comments clarifying A-sync case calculations.

Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>