Add a pci_scan_root_bus_msi() interface so an arch can specify the MSI
controller up front.  This removes the need for a pcibios callback to set
the MSI controller later.

This is not exported because I'd like to replace the variety of "scan root
bus" interfaces with a single, more extensible interface that can handle
the MSI controller, domain, pci_ops, resources, etc.  I hope this interface
is temporary.

[bhelgaas: changelog, split into separate patch]
Suggested-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Jingoo Han <jingoohan1@gmail.com>