peppy: Enable power limiting for thermal control

Limit power to 12W at 73C and remove limit at 68C.

To have the CPU consume maximum power it is necessary to stress
both the CPU and the GPU.  Bastion (chrome.supergiantgames.com)
and/or webglsamples.googlecode.com can be useful for this.

Testing this properly requires a script to report the running
average power readings.  The watch_power.sh script is attached
to this issue in the partner tracker.

1) Run watch_power.sh continuously:
localhost ~ # watch -n 0 bash -e /tmp/watch_power.sh
2) Start Bastion (or other stress apps).  The power draw should
be close to 15W if under enough load.
3) Watch until temperature climbs above 73C and is caught by
the thermal zone 10 second poll, this can be sped up by blocking
or removing the fan.
4) The ACPI thermal zone states should change to reflect that
active[2] is now enabled and power consumption should drop to 12W.
5) Stop the stress apps and wait until the CPU cools off again,
enable the fan again if it was removed.
6) The ACPI thermal zone state should switch back to active[3].

Change-Id: Ie6714a8543d4f06edf8513086fc9c968273bdb23
Signed-off-by: Duncan Laurie <dlaurie@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/60545
Reviewed-on: http://review.coreboot.org/4335
Tested-by: build bot (Jenkins)
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

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