arm: rmobile: alt: Change clock of SCIF to external clock

Change clock of SCIF for Alt board is used to external clock.
This changes to using external clock.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>