arm64: Force SSBS on context switch

On a CPU that doesn't support SSBS, PSTATE[12] is RES0.  In a system
where only some of the CPUs implement SSBS, we end-up losing track of
the SSBS bit across task migration.

To address this issue, let's force the SSBS bit on context switch.

Change-Id: Ifb8836b7a616db5fd21f6de780693dd9f97adad9
Fixes: 8f04e8e6e29c ("arm64: ssbd: Add support for PSTATE.SSBS rather than trapping to EL3")
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Git-commit: 11ce805228fe1ac14225b4886fb396b485981263
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git


[neeraju@codeaurora: Resolve trivial merge conflicts]
Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>