sh: sh2: Change the specification method of IRQ to SCIx_IRQ_MUXED

Some SCIF devices specify the same IRQ. We can use SCIx_IRQ_MUXED for this.
This is correction to the SH2/SH2A series.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>