docs: cpufreq: fix a broken reference

The old cpufreq docs got moved. Let's also update its
pointer inside Kconfig.x86.

Fixes: 03b22496 ("Documentation: cpufreq: Move legacy driver documentation")
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>