thermal: cpu_cooling: improve documentation for get_cpu_frequency

Fix kernel-doc warning on get_cpu_frequency and improve
documentation comments.

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>