Zeus: apply the MSR SSBS instruction

Zeus supports the SSBS mechanism and also the new MSR instruction to
immediately apply the mitigation. Hence, the new instruction is utilised
in the Zeus-specific reset function.

Change-Id: I962747c28afe85a15207a0eba4146f9a115b27e7
Signed-off-by: John Tsichritzis <john.tsichritzis@arm.com>