[CPUFREQ] S5PV210: Add reboot notifier to prevent system hang

When system reboot, the CPUFREQ level should be 800MHz to prevent
system lockup.

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>