dma: coh901318: push header down into the DMA subsystem

The defines in U300's <mach/coh901318.h> are now only used by
the DMA engine driver itself, so move the header down into the
DMA subsystem.

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