sh: SH7760 DMABRG support.

The DMABRG is a special DMA unit within the SH7760 which does data
transfers from main memory to Audio units and USB shared memory.
It has 3 IRQ lines which generate 10 events, which have to be masked
unmasked and acked in a single 32bit register. It works independently
from the tradition SH DMAC, but blocks usage of DMAC channel 0.

This patch adds 2 functions to associate callbacks with DMABRG events
and initialization.

Signed-off-by: Manuel Lauss <mano@roarinelk.homelinux.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>