defconfig: 8937/8917/8940: Move RCU callbacks processing to kernel thread

Under high load situations RCU callback processing adds a
significant workload to softirq. This may result in CPU
starvation for other time critical entities, waiting to run
in softirq context. This patch eanbles the kernel option
which allows to perform RCU callback processing in separate
kernel threads and thus reducing the softriq workload a lot.

Change-Id: Ic8e31110c29a1b87b7eee00e59e656b7d1652d4d
Signed-off-by: Imran Khan <kimran@codeaurora.org>