cpufreq_dt: Do not attach the device if the cpu isn't present

If we boot with hw.ncpu=X (available on arm and arm64 at least) we
shouldn't attach the cpufreq driver as cf_set_method will try to get
the cpuid and it doesn't exists.
This solves cpufreq panicing on RockChip RK3399 when booting with
hw.ncpu=4

MFC after: 1 week