powerpc: Define NR_CPUS all the time

include/linux/threads.h defines a default value for CONFIG_NR_CPUS
but suggests the architectures should always define it.

So do it, when CONFIG_SMP is not selected set CONFIG_NR_CPUS to 1.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/afef0ec65a8ba8651bf4f6e4f4f08a8b6991dbfb.1620379469.git.christophe.leroy@csgroup.eu