sh: sh4a: 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.
And change use to evt2irq(), without specifying the value of IRQ directly.
This is correction to the SH4A series.

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