PCI: Add devm_pci_alloc_host_bridge() interface

Struct pci_host_bridge can be allocated by PCI host bridge drivers which
usually allocate and map memory through devm managed interfaces.

Add a devm version for the pci_alloc_host_bridge() interface to simplify
PCI host controller driver porting and simplify the driver failure paths.

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