arm64: defconfig: Enable CONFIG_WQ_POWER_EFFICIENT_DEFAULT

commit cee22a15 ("workqueues: Introduce new flag WQ_POWER_EFFICIENT
for power oriented workqueues") introduced the concept of power
efficient workqueues (4 years back), but it was never enabled in
upstream kernel configs.

Power efficient workqueues are simply marked as "unbound," so that jobs
queued to them can run on any CPU in the system. It leaves the target
CPU selection to the scheduler, which is the best place for such
decision making. This improves power efficiency for workqueues which are
otherwise pinned to a CPU.

Enable it for ARM64 platforms as ARM platforms were the main target for
the introduction of power efficient workqueues.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>