cpufreq: Split cpufreq_online()

In preparation for the introduction of cpufreq policy locking guards,
move the part of cpufreq_online() that is carried out under the policy
rwsem into a separate function called cpufreq_policy_online().

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://patch.msgid.link/3354747.aeNJFYEL58@rjwysocki.net