dma: coh901318: add devicetree support

This adds support for probing the COH 901 318 DMA controller
and channels from the device tree.

Contains portions of a sketch patch from Arnd Bergmann.

Cc: Arnd Bergmann <arnd@arndb.de>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>