kernel: smp: trylock when stopping CPU

On current Falcon platform, we have 4 CPUs. When one CPU panic and issue
IPI_CPU_STOP, the other three CPUs may get triggered to handle ipi_cpu_stop
and hold stop_lock spin lock at the same time, which would introduce spin
lockup finally.

The change uses spin_trylock instead to avoid the other CPUs lockup when panic

Change-Id: I25642f83143e4078e86cdb0fc9a2c6f04bfa1cd6
Signed-off-by: Hong-Mei Li <a21834@motorola.com>
Signed-off-by: Jake Weinstein <xboxlover360@gmail.com>