[MIPS] Fix declaration of smp_prepare_cpus() platform hook.

    
A while ago prom_prepare_cpus was replaced by plat_prepare_cpus but
the declaration has stayed unchanged.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

include/asm-mips/smp.h

@@ -85,9 +85,9 @@ extern void prom_init_secondary(void);
 extern void plat_smp_setup(void);
 
 /*
- * Called after init_IRQ but before __cpu_up.
+ * Called in smp_prepare_cpus.
  */
-extern void prom_prepare_cpus(unsigned int max_cpus);
+extern void plat_prepare_cpus(unsigned int max_cpus);
 
 /*
  * Last chance for the board code to finish SMP initialization before