Documentation: admin-guide: pm: Document intel-speed-select

Added documentation to configure servers to use Intel(R) Speed
Select Technology using intel-speed-select tool.

Signed-off-by: Srinivas Pandruvada <[email protected]>
Acked-by: Andriy Shevchenko <[email protected]>
Signed-off-by: Rafael J. Wysocki <[email protected]>

Author: spandruvada