cpufreq: Add warning message to powernow-k8

cpufreq modules are often loaded from init scripts that assume that
all recent AMD systems will use powernow-k8.
To inform the user of the change of support and ease the transition
to acpi-cpufreq, emit a warning message.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>