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

Signed-off-by: Arto Merilainen <amerilainen@nvidia.com>
Tested-by: Sang-Hun Lee <sanlee@nvidia.com>
Reviewed-by: Kevin Huang (Eng-SW) <kevinh@nvidia.com>
Reviewed-by: Mitch Luban <mluban@nvidia.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>