cpufreq: Kconfig: fix documentation links

User documentation for cpufreq governors and drivers has been moved to
admin-guide; adjust references from Kconfig entries accordingly.

Remove references from undocumented cpufreq drivers, as well as the
'userspace' cpufreq governor, for which no additional details are
provided in the admin-guide text.

Fixes: 2a0e4927 ("cpufreq: User/admin documentation update and consolidation")
Signed-off-by: Alexander Monakov <amonakov@ispras.ru>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>