If we trigger the bus rescan from sysfs, we'll try to lock the PCI
rescan mutex recursively and deadlock - the platform device will be
populated and probed on the same thread that handles the sysfs write.

Add a workqueue to the pwrctl code on which we schedule the rescan for
controlled PCI devices. While at it: add a new interface for
initializing the pwrctl context where we'd now assign the parent device
address and initialize the workqueue.

Fixes: 4565d265 ("PCI/pwrctl: Add PCI power control core code")
Reported-by: Konrad Dybcio <konradybcio@kernel.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>