powerpc/pseries/lparcfg: Use num_possible_cpus() for potential processors

The systemcfg processorCount variable tracks currently online variables,
not possible ones, so the stored value is wrong.
The code preferably tries to use the ibm,lrdr-capacity field 4 which
"represents the maximum number of processors that the guest can have."
Switch from processorCount to the better matching num_possible_cpus().

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241010-vdso-generic-base-v1-24-b64f0842d512@linutronix.de