ARM: u300: add the COH 901 318 DMAC to device tree

This adds the COH 901 318 DMA controller to the U300
device tree. All devices now converted to device tree
so far will start to find their DMA channels.

Note that the U300 is not yet using the device tree
to obtain DMA channels, but this is a first step.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>