Add separate refereces sections to the cpufreq.rst and
intel_pstate.rst documents under admin-quide/pm and list the
references to external documentation in there.

Update the ACPI specification URL while at it.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>