arm64: Provide a command line to disable spectre_v2 mitigation

euler inclusion
category: feature
bugzilla: 11011
CVE: NA

Patch will be in mainlien kernel 5.2
--------------------------------------------------

There are various reasons, including bencmarking, to disable spectrev2
mitigation on a machine. Provide a command-line to do so.

Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
[Hanjun: fix conflicts which we don't have PPC PPC_FSL_BOOK3E
 arch spectre-v2 mitigation in 4.19 kernel, so remvoe PPC_FSL_BOOK3E]
Conflicts:
	Documentation/admin-guide/kernel-parameters.txt
Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
Reviewed-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>