arm64: kernel: Update the stale comment

Commit af391b15 ("arm64: kernel: rename __cpu_suspend to keep it aligned with arm")
has used @index instead of @arg, but the comment is stale, update it.

Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Link: https://lore.kernel.org/r/1620280462-21937-1-git-send-email-zhangshaokun@hisilicon.comSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/kernel/cpuidle.c

@@ -29,7 +29,7 @@ int arm_cpuidle_init(unsigned int cpu)
 
 /**
  * arm_cpuidle_suspend() - function to enter a low-power idle state
- * @arg: argument to pass to CPU suspend operations
+ * @index: argument to pass to CPU suspend operations
  *
  * Return: 0 on success, -EOPNOTSUPP if CPU suspend hook not initialized, CPU
  * operations back-end error code otherwise.