[ARM] S3C: Make common <plat/dma.h> header

Move the common information from the <mach/dma.h> header into a new
shareable <plat/dma.h> header for re-use on the s3c64xx.

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