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

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

Based on earlier discussions[1] we attempted to find a suitable
location for the omap DMA header in commit 2b6c4e73 (ARM: OMAP:
DMA: Move plat/dma.h to plat-omap/dma-omap.h) until the conversion
to dmaengine is complete.

Unfortunately that was before I was able to try to test compile
of the ARM multiplatform builds for omap2+, and the end result
was not very good.

So I'm creating yet another all over the place patch to cut the
last dependency for building omap2+ for ARM multiplatform. After
this, we have finally removed the driver dependencies to the
arch/arm code, except for few drivers that are being worked on.

The other option was to make the <plat-omap/dma-omap.h> path
to work, but we'd have to add some new header directory to for
multiplatform builds.

Or we would have to manually include arch/arm/plat-omap/include
again from arch/arm/Makefile for omap2+.

Neither of these alternatives sound appealing as they will
likely lead addition of various other headers exposed to the
drivers, which we want to avoid for the multiplatform kernels.

Since we already have a minimal include/linux/omap-dma.h,
let's just use that instead and add a note to it to not
use the custom omap DMA functions any longer where possible.

Note that converting omap DMA to dmaengine depends on
dmaengine supporting automatically incrementing the FIFO
address at the device end, and converting all the remaining
legacy drivers. So it's going to be few more merge windows.

[1] https://patchwork.kernel.org/patch/1519591/#

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Signed-off-by: Tony Lindgren <tony@atomide.com>