dma: shdma: NMI support.

Presently DMA transfers are interrupted and aborted by the NMI. This
implements some basic logic for more gracefully handling and clearing
each controller's NMIF flag via the NMI die chain, needed to resume
transfers post-NMI.

Reported-by: Michael Szafranek <Michael.Szafranek@emtrion.de>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>