powerpc/cpumask: Dynamically allocate cpu_sibling_map and cpu_core_map cpumasks

Dynamically allocate cpu_sibling_map and cpu_core_map cpumasks.

We don't need to set_cpu_online() the boot cpu in smp_prepare_boot_cpu,
init/main.c does it for us.

We also postpone setting of the boot cpu in cpu_sibling_map and cpu_core_map
until when the memory allocator is available (smp_prepare_cpus), similar
to x86.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>