[MIPS] SMP: Fix initialization order bug.

A recent change requires cpu_possible_map to be initialized before
smp_sched_init() but most MIPS platforms were initializing their
processors in the prom_prepare_cpus callback of smp_prepare_cpus.  The
simple fix of calling prom_prepare_cpus from one of the earlier SMP
initialization hooks doesn't work well either since IPIs may require
init_IRQ() to have completed, so bit the bullet and split
prom_prepare_cpus into two initialization functions, plat_smp_setup
which is called early from setup_arch and plat_prepare_cpus called where
prom_prepare_cpus used to be called.
    
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/smp.c

@@ -236,7 +236,7 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 	init_new_context(current, &init_mm);
 	current_thread_info()->cpu = 0;
 	smp_tune_scheduling();
-	prom_prepare_cpus(max_cpus);
+	plat_prepare_cpus(max_cpus);
 }
 
 /* preload SMP state for boot cpu */