Add support for Intel Turbo Boost feature

Add support for Intel Turbo Boost feature

From wikipedia:
Intel Turbo Boost is a technology implemented by Intel in certain
versions of their Nehalem- and Sandy Bridge-based CPUs, including Core
i5 and Core i7 that enables the processor to run above its base
operating frequency via dynamic control of the CPU's "clock rate".
It is activated when the operating system requests the highest
performance state of the processor.

Change-Id: I166ead7c219083006c2b05859eb18749c6fbe832
Signed-off-by: Duncan Laurie <dlaurie@google.com>
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/844
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>