MIPS: CPS: Fix potential NULL pointer dereferences in cps_prepare_cpus()

Check the return values of kcalloc() and exit early to avoid potential
NULL pointer dereferences.

Compile-tested only.

Cc: stable@vger.kernel.org
Fixes: 75fa6a58 ("MIPS: CPS: Introduce struct cluster_boot_config")
Fixes: 0856c143 ("MIPS: CPS: Boot CPUs in secondary clusters")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>