PCI: Add pci_free_host_bridge() interface

Commit a52d1443

 ("PCI: Export host bridge registration interface")
exported the pci_alloc_host_bridge() interface so that PCI host controllers
drivers can make use of it.

Introduce pci_alloc_host_bridge() kernel counterpart to free the host
bridge data structures, pci_free_host_bridge(), export it and update kernel
functions releasing host bridge objects allocated memory to make use of it.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Arnd Bergmann <arnd@arndb.de>