arm64: correct the effect of mitigations off on kpti

If KASLR is enabled, then kpti will be forced to be enabled even if
mitigations off, so we need to adjust the description of this parameter.

Signed-off-by: Liu Song <liusong@linux.alibaba.com>
Link: https://lore.kernel.org/r/1656033648-84181-1-git-send-email-liusong@linux.alibaba.com


Signed-off-by: Will Deacon <will@kernel.org>