[power] Add hooks for runtime device power control.

[power] Add hooks for runtime device power control.

dpm_runtime_{suspend,resume} control the power state of a single device
while the system is running.

dpm_runtime_suspend() will save state of the device, then attempt to power
it down. This happens with interrupts enabled, so if the device does not
support that, the device's state is restored, and we continue on our merry
way.

dpm_runtime_resume() powers the device back on, then restores state of the
device.

dpm_set_power_state() simply notifies the core of the power state the
device is in. Drivers can use this, since they are the only ones that can
really tell.