ARM: SAMSUNG: Remove dma-plat.h to allow plat-s3c64xx to be removed

dma-plat.h is the last file left in plat-s3c64xx, but to remove it we
must also change the use of dma-plat.h by the core code and the s3c24xx
implementation.

Rename the s3c24xx dma-plat.h in the common plat-samsung directory as it
may be used for other ports. Move the specific dma bits into the
mach-s3c64xx directory and update the build as needed.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>