configs: Enable PIDs cgroup

My use case to is to allow Kubernetes master to run on Raspberry Pi 3.
Kubernetes introduced [Pid limiting](https://github.com/kubernetes/enhancements/blob/master/keps/sig-node/20190129-pid-limiting.md),
on v.1.14.0 which includes [Pod Pids Limit](kubernetes/kubernetes@bce9d5f)
and [Node Pids Limit](kubernetes/kubernetes@2597a1d#diff-fa76bb6ae2d9b4bb3d023737fe5e6029R333)
that requires pids cgroup.

See: #2968

arch/arm/configs/bcm2709_defconfig

@@ -20,6 +20,7 @@ CONFIG_CGROUP_FREEZER=y
 CONFIG_CPUSETS=y
 CONFIG_CGROUP_DEVICE=y
 CONFIG_CGROUP_CPUACCT=y
+CONFIG_CGROUP_PIDS=y
 CONFIG_NAMESPACES=y
 CONFIG_USER_NS=y
 CONFIG_SCHED_AUTOGROUP=y