sh: define DMA slaves per CPU type, remove now redundant header

Now that DMA slave IDs are only used used in platform specific code and have
become opaque cookies for the rest of the code, we can make the, CPU specific
too.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>