init: use unbound instead of highpriority wq in early init

High priority workqueue is cpu bound so work on it cannot
flow to other cores freely afterwards. And linux tends to
use current core where queue_work_on is called. So finally
all subsystems may end up running on the same core. This
is not what we expect. So we need to use system_unbound_wq
instead. This way, all subsystems can flow freely on cpu
cores based on work load and we can achieve better cpu
parallelism.

Change-Id: Ic474cc4df55769154630f9e106fd786084fab48b
Signed-off-by: Jiacheng Zheng <jiaczhen@codeaurora.org>