PCI: Add pci_scan_root_bus_bridge() interface

The current pci_scan_root_bus() interface is made up of two main code
paths:

  - pci_create_root_bus()
  - pci_scan_child_bus()

pci_create_root_bus() is a wrapper function that allows to create a struct
pci_host_bridge structure, initialize it with the passed parameters and
register it with the kernel.

As the struct pci_host_bridge require additional struct members,
pci_create_root_bus() parameters list has grown in time, making it unwieldy
to add further parameters to it in case the struct pci_host_bridge gains
more members fields to augment its functionality.

Since PCI core code provides functions to allocate struct pci_host_bridge,
instead of forcing the pci_create_root_bus() interface to add new
parameters to cater for new struct pci_host_bridge functionality, it is
more suitable to add an interface in PCI core code to scan a PCI bus
straight from a struct pci_host_bridge created and customized by each
specific PCI host controller driver.

Add a pci_scan_root_bus_bridge() function to allow PCI host controller
drivers to create and initialize struct pci_host_bridge and scan the
resulting bus.

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