MFC r355360-r355361

r355360:
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

r355361:
cpufreq_dt: Do not fetch again hw.ncpu

MD code already set the global variable mp_ncpus according to
the tunable hw.ncpu so use the global variable directly.

Reported by: ian