PM / devfreq: drop comment about thermal setting max_freq

The thermal infrastructure should use the devfreq cooling device, which
uses the OPP library to disable OPPs as necessary.

Fix a couple of typos in the same comment while we are at it.

Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>