cpufreq: Add defensive check during driver registration

Currently, cpufreq allows drivers to implement both ->target() and
->target_index() callbacks, but that can lead to ambiguous or incorrect
behavior.

For this reason, prevent cpufreq drivers implementing both ->target()
and ->target_index() at the same time from registering.

This check can help to catch driver implementation mistakes early and
improve overall robustness, without affecting existing valid drivers.

Signed-off-by: Zihuan Zhang <zhangzihuan@kylinos.cn>
Link: https://patch.msgid.link/20250908085738.31602-1-zhangzihuan@kylinos.cn


[ rjw: Subject adjustment and changelog rewrite ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>