dmaengine: pl330: flush before wait, and add dev burst support.

Do DMAFLUSHP _before_ the first DMAWFP to ensure controller
and peripheral are in agreement about dma request state before first
transfer.  Add support for burst transfers to/from peripherals. In the new
scheme, the controller does as many burst transfers as it can then
transfers the remaining dregs with either single transfers for
peripherals, or with a reduced size burst for memory-to-memory transfers.

Signed-off-by: Frank Mori Hess <fmh6jj@gmail.com>
Tested-by: Frank Mori Hess <fmh6jj@gmail.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>