dmaengine: pl08x: support dt channel assignment

Add support for assigning DMA channels from a device tree.

[je: remove channel sub-node parsing, dynamic channel creation on xlate]

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>