docs: thermal: convert cpu-idle-cooling.rst to ReST

docs: thermal: convert cpu-idle-cooling.rst to ReST

Despite being named with .rst extension, this file doesn't
match the ReST standard. It actually causes a crash at
Sphinx:

	Sphinx parallel build error:
	docutils.utils.SystemMessage: /devel/v4l/docs/Documentation/driver-api/thermal/cpu-idle-cooling.rst:69: (SEVERE/4) Unexpected section title.

Add needed markups for it to be properly parsed.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/7640755514809a7b5fe2756f3702613865877dcb.1592203650.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>