PCI: Add support for unbinding the generic PCI host controller

Add support for unbinding the generic PCI host controller.  This is
particularly useful when working in virtual environments where the
controller may come and go, but possibly not only there.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Will Deacon <will.deacon@arm.com>
CC: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>