thermal: cpu_cooling: update documentation for cpufreq_get_max_state

Update documentation for cpufreq_get_max_state callback.

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>