PCI: Add pcibios_release_device()

Platforms may want to provide architecture-specific functionality when
a PCI device is released.  Add a pcibios_release_device() call that
architectures can override to do so.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>