thermal: cpu_cooling: document node in struct cpufreq_cooling_device

thermal: cpu_cooling: document node in struct cpufreq_cooling_device

The node field of struct cpufreq_cooling_device was reintroduced in
2dcd851fe4b4 (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 <[email protected]>
Cc: Eduardo Valentin <[email protected]>
Cc: Zhang Rui <[email protected]>
Signed-off-by: Javi Merino <[email protected]>
Signed-off-by: Zhang Rui <[email protected]>