thermal: cpu_cooling: document node in struct cpufreq_cooling_device

The node field of struct cpufreq_cooling_device was reintroduced in
2dcd851fe4 (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>

drivers/thermal/cpu_cooling.c

@@ -39,6 +39,7 @@
  * @cpufreq_val: integer value representing the absolute value of the clipped
  *	frequency.
  * @allowed_cpus: all the cpus involved for this cpufreq_cooling_device.
+ * @node: list_head to link all cpufreq_cooling_device together.
  *
  * This structure is required for keeping information of each
  * cpufreq_cooling_device registered. In order to prevent corruption of this a