[CPUFREQ][3/4] cpufreq "cpu group" awareness: do symlinks for other CPUs instead of registering kobjects

Only register each "affected_cpu_mask" once, and do symlinks for those
CPUs not directly registered to the driver model.

Zwane Mwaikambo tested it, and here is the result:

root@morocco cpu {0:0} pwd
/sys/devices/system/cpu
root@morocco cpu {0:0} tree
.
|-- cpu0
|   |-- cpufreq
|   |   |-- cpuinfo_cur_freq
|   |   |-- ...
|   |   -- scaling_setspeed
|   -- online
-- cpu1
    |-- cpufreq -> ../../../../devices/system/cpu/cpu0/cpufreq
    -- online
Signed-off-by: Dominik Brodowski <linux@brodo.de>
Signed-off-by: Dave Jones <davej@redhat.com>