arm64: smp: Treat unknown boot failures as being 'stuck in kernel'

When we fail to bring a secondary CPU online and it fails in an unknown
state, we should assume the worst and increment 'cpus_stuck_in_kernel'
so that things like kexec() are disabled.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>