Revert "parisc: Only list existing CPUs in cpu_possible_mask"

This reverts commit 0921244f.

It broke CPU hotplugging because it modifies the __cpu_possible_mask
after bootup, so that it will be different than nr_cpu_ids, which
then effictively breaks the workqueue setup code and triggers crashes
when shutting down CPUs at runtime.

Guenter was the first who noticed the wrong values in __cpu_possible_mask,
since the cpumask Kunit tests were failig.

Reverting this commit fixes both issues, but sadly brings back this
uncritical runtime warning:
register_cpu_capacity_sysctl: too early to get CPU4 device!

Signed-off-by: Helge Deller <deller@gmx.de>
Reported-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lkml.org/lkml/2024/2/4/146
Link: https://lore.kernel.org/lkml/Zb0mbHlIud_bqftx@slm.duckdns.org/t/
Cc: stable@vger.kernel.org # 6.0+