[CPUFREQ] S5PV210: Lock a mutex while changing the cpu frequency

Without this lock the call to change the frequency for suspend could
switch to a new frequency while another thread was still changing the
cpu voltage.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Dave Jones <davej@redhat.com>