thermal: cpu_cooling: document node in struct cpufreq_cooling_device

The node field of struct cpufreq_cooling_device was reintroduced in
2dcd851f

 (thermal: cpu_cooling: Update always cpufreq policy with
thermal constraints) but without the documentation that it once had.
Add it back so that all the fields of struct cpufreq_cooling_device
are documented.

Cc: Yadwinder Singh Brar <yadi.brar@samsung.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Javi Merino <javi.merino@arm.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>