PCI: make pci_bus a struct device

This moves the pci_bus class device to be a real struct device and at
the same time, place it in the device tree in the correct location.

Note, the old "bridge" symlink is now gone, but this was a non-standard
link and no userspace program used it.  If you need to determine the
device that the bus is on, follow the standard device symlink, or walk
up the device tree.

Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>