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.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>