sh: Fix DMAC base address for SH7709S

On SH7709S, DMAC can be found at 0xa4000020 (as with most of
the other sh3 cpu subtypes).

Split out definition of DMAC base address from definitions of
DMTE irqs.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>