Documentation/cpu-freq: add intel-pstate.txt

The Intel P-state driver is currently undocumented. Add some
documentation based on the cover-letter sent with the original series.

Cc: Dirk Brandewie <dirk.brandewie@gmail.com>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Linux PM <linux-pm@vger.kernel.org>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>