msm: thermal: Clean up msm cpufreq function calls

Remove msm_cpufreq_set_freq_limits and register with
CPUFREQ_POLICY_NOTIFIER to get immediate update on cpu frequencies
as long as a cpufreq_update_policy is called. Ensure that when
cpufreq_update_policy is called, cpu will not be hotplugged by
calling get_online_cpus().

Change-Id: I802d51ad1c352a19d2e456f9f6a89f8d5b89aad8
Signed-off-by: Jennifer Liu <chihning@codeaurora.org>