thermal: cpu_cooling: update kernel-doc for cpufreq_cooling_register

Add proper documentation for exported function cpufreq_cooling_register.

Signed-off-by: Eduardo Valentin <eduardo.valentin@ti.com>
Acked-by: Amit Daniel Kachhap <amit.daniel@samsung.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>