sh: Fix up number of on-chip DMA channels on SH7091.

This accidentally regressed when the multi-IRQ changes went in,
switching SH7091 from 4 to 6 channels. Add SH7091 back in to the
4-channel dependency list.

Reported-by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>