PM: EM: Fix incorrect description of the cost field in struct em_perf_state

PM: EM: Fix incorrect description of the cost field in struct em_perf_state

Due to commit 1b600da51073 ("PM: EM: Optimize em_cpu_energy() and remove
division"), the logic for energy consumption calculation has been modified.
The actual calculation of cost is 10 * power * max_frequency / frequency
instead of power * max_frequency / frequency.

Therefore, the comment for cost has been updated to reflect the correct
content.

Fixes: 1b600da51073 ("PM: EM: Optimize em_cpu_energy() and remove division")
Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
[ rjw: Added Fixes: tag ]
Link: https://patch.msgid.link/20251230061534.816894-1-tianyaxiong@kylinos.cn
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>