PCI: mvebu: Remove mvebu_pcie_scan_bus()

After b97ea289

 ("PCI: Assign resources before drivers claim devices
(pci_scan_root_bus())"), pci_scan_root_bus() no longer adds the devices, so
it is equivalent to mvebu_pcie_scan_bus().

Remove mvebu_pcie_scan_bus() (the hw.scan method), so we use the generic
pci_scan_root_bus() path.  We also need to use pci_common_init_dev()
instead of pci_common_init() so we can supply the host bridge device
pointer.

[bhelgaas: changelog]
Tested-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
CC: Jason Cooper <jason@lakedaemon.net>