[CPUFREQ] S5PV210: Add pm_notifier to prevent system unstable

Minimum 800MHz is needed to enter/exit suspend mode due to voltage mismatch.

Signed-off-by: Huisung Kang <hs1218.kang@samsung.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>