arm64: ssbd: Add support for PSTATE.SSBS rather than trapping to EL3

On CPUs with support for PSTATE.SSBS, the kernel can toggle the SSBD
state without needing to call into firmware.

This patch hooks into the existing SSBD infrastructure so that SSBS is
used on CPUs that support it, but it's all made horribly complicated by
the very real possibility of big/little systems that don't uniformly
provide the new capability.

Change-Id: I4d195d50bdb04fbd42ad12cf0f9b3d990b686b5a
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: 8f04e8e6e29c93421a95b61cad62e3918425eac7
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Mayank Grover <groverm@codeaurora.org>