ARM: shmobile: r8a7779: scif .irqs used SCIx_IRQ_MUXED()

This patch tidyup scif .irqs settings by using
SCIx_IRQ_MUXED() macro.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>