admin-guide: pm: intel_epb: Add SPDX license tag and copyright notice

Add an SPDX license tag and a copyright notice to the intel_epb.rst
file under Documentation/admin-quide/pm.

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