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>
dma: edma: Add support for Cyclic DMA

Using the PaRAM configuration function that we split for reuse by the
different DMA types, we implement Cyclic DMA support.
For the cyclic case, we pass different configuration parameters to this
function, and handle all the Cyclic-specific functionality separately.

Callbacks to the DMA users are handled using vchan_cyclic_callback in
the virt-dma layer. Linking is handled the same way as the slave SG case
except for the last slot where we link it back to the first one in a
cyclic fashion.

For continuity, we check for cases where no.of periods is great than the
MAX number of slots the driver can allocate for a particular descriptor
and error out on such cases.

Signed-off-by: Joel Fernandes <joelf@ti.com>
dma: edma: Increase maximum SG limit to 20

davinci-pcm uses 16 as the no.of periods. With this, in EDMA we have to
allocate atleast 17 slots: 1 slot for channel, and 16 slots the periods.

Due to this, the MAX_NR_SG limitation causes problems, set it to 20 to make
cyclic DMA work when davinci-pcm is converted to use DMA Engine.

Signed-off-by: Joel Fernandes <joelf@ti.com>