doc: dt: dma: add binding doc for pl08x

This introduces device tree bindings for the PL08x DMA controllers
when used with fixed signal assignment per channel, i.e. if each
channel on the PL08x is assigned precisely one burst/single signal
set.

[je: remove channel sub-node parsing, use cell value to assign AHB]

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>