cpu_kill currently returns one for success and zero for failure, which
is unlike all the other cpu_operations, which return zero for success
and an error code upon failure. This difference is unnecessarily
confusing.

Make cpu_kill consistent with the other cpu_operations.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Acked-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: NHanjun Guo <hanjun.guo@linaro.org>
Tested-by: NHanjun Guo <hanjun.guo@linaro.org>
Cc: Will Deacon <will.deacon@arm.com>