ARM: OMAP: DMA: Move plat/dma.h to plat-omap/dma-omap.h

Move plat/dma.h to plat-omap/dma-omap.h as part of single
zImage work

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>