kernel: power: Add PM_USERSPACE_FROZEN workqueue

Some device drivers require a callback to be called after the userspace
processes are frozen. This patch adds PM_USERSPACE_FROZEN workqueue
which is called after userspace processes are frozen but when the
kernel threads are still functioning.

Bug 1344551

Change-Id: I0e6fd7e2473db168d01c88bc0192326ceea92ebe
Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Reviewed-on: http://git-master/r/266774
(cherry picked from commit d964493291ef87eea1a2ee47b5b66305bb18bcf3)
Reviewed-on: http://git-master/r/274939
Tested-by: Sang-Hun Lee <sanlee@nvidia.com>
Reviewed-by: Kevin Huang (Eng-SW) <kevinh@nvidia.com>
Reviewed-by: Mitch Luban <mluban@nvidia.com>

Conflicts:
	include/linux/suspend.h