[CPUFREQ] speedstep-smi: it works on at least one P4M

The speedstep-smi driver actually works on >=1 notebook with a
Pentium 4-M CPU where all other cpufreq drivers fail. Therefore,
allow speedstep-smi on P4Ms again, but warn users of likely failure

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Dave Jones <davej@redhat.com>