SH: constify multiple DMA related objects and references to them

Lists of DMA channels and slaves are not changed, make them constant. Besides,
SH7724 channel and slave configuration of both DMA controllers is identical,
remove the extra copy of the configuration data.

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