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: Martin Schwidefsky <schwidefsky@de.ibm.com>