arm64: Add 'ssbd' command-line option

On a system where the firmware implements ARCH_WORKAROUND_2,
it may be useful to either permanently enable or disable the
workaround for cases where the user decides that they'd rather
not get a trap overhead, and keep the mitigation permanently
on or off instead of switching it on exception entry/exit.

In any case, default to the mitigation being enabled.

Change-Id: I66f7b80ad2c0d6acf8ce18d7035bbb56a3146af6
Reviewed-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Git-commit: a43ae4dfe56a01f5b98ba0cb2f784b6a43bafcc6
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[isaacm@codeaurora.org: remove code that is not needed to
 apply workaround for ssbd. Also, move ssbd constant
 definitions, to expose these constants to assembly,
 so compilation does not break later.]
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>