kernel: Restrict unbound kthreads to little CPU cluster

Kernel threads tend to be rather low in overhead, so some power savings
can be reaped by restricting as many kernel threads as possible to the
little (low-power) CPU cluster.

Signed-off-by: Sultanxda <sultanxda@gmail.com>
Signed-off-by: Francisco Franco <franciscofranco.1990@gmail.com>