SH: extend SCI DMA support to work on SCIFA ports

SCIFA ports have additional bits to control DMA requests and they must have
respective interrupt sources enabled, as the datasheet suggests, the only way
to avoid actually taking interrupts in addition to DMA events is by masking the
IRQ on the CPU.

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