dmaengine: omap-dma: provide a hook to get the underlying DMA platform ops

Provide and use a hook to obtain the underlying DMA platform operations
so that omap-dma.c can access the hardware more directly without
involving the legacy DMA driver.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>