powerpc/prom: Fix 32-bit build

Add an IS_ENABLED() check to fix the build error:

arch/powerpc/kernel/prom.o: in function `early_init_dt_scan_cpus':
  prom.c:(.init.text+0x2ea): undefined reference to `boot_cpu_node_count'

Fixes: e13d23a4 ("powerpc: export the CPU node count")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>