parisc: Only list existing CPUs in cpu_possible_mask

The inventory knows which CPUs are in the system, so this bitmask should
be in cpu_possible_mask instead of the bitmask based on CONFIG_NR_CPUS.

Reset the cpu_possible_mask before scanning the system for CPUs, and
mark each existing CPU as possible during initialization of that CPU.

This avoids those warnings later on too:

 register_cpu_capacity_sysctl: too early to get CPU4 device!

Signed-off-by: Helge Deller <deller@gmx.de>
Noticed-by: John David Anglin <dave.anglin@bell.net>