arm64: smp: add missing completion for secondary boot

Commit 149c2415 ("ARM: SMP: use a timing out completion for cpu
hotplug") modified arm's CPU up path to use completions. It seems that
we only got half of this patch for arm64, so add the missing call to
complete.
Reported-by: Jon Brawn <jon.brawn@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/smp.c

@@ -211,8 +211,7 @@ asmlinkage void __cpuinit secondary_start_kernel(void)
 	 * before we continue.
 	 */
 	set_cpu_online(cpu, true);
-	while (!cpu_active(cpu))
-		cpu_relax();
+	complete(&cpu_running);
 
 	/*
 	 * OK, it's off to the idle thread for us