arm: rmobile: lager: Fix change of the CPU frequency

The change of the CPU frequency is waited for until PLL0ST of the PLLECR is
set to 1.

Signed-off-by: Hisashi Nakamura <hisashi.nakamura.ak@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>