Since the device subsystem contains a global list of registered objects,
we don't need to keep our own separate list, so we axe it. This also
results in a savings of sizeof(void *) x 2 in the size of each device.

drivers/base/power.c is updated to take the subsystem's rwsem and iterate
over that list of objects.

This leaves only the children list of struct device protected by the driver
model semaphore. This could be removed, but there is one user of it (EDD), 
so it can't happen yet.

This patch also removes the driver model spinlock, which isn't used anymore.