PCI: dwc: keystone: Convert .scan_bus() callback to use add_bus

PCI: dwc: keystone: Convert .scan_bus() callback to use add_bus

TI keystone is the only Designware driver using .scan_bus(). This
function pointer is the only thing preventing the Designware driver from
using pci_host_probe(). Let's use the pci_ops.add_bus hook instead.

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Rob Herring <[email protected]>
Signed-off-by: Lorenzo Pieralisi <[email protected]>
Cc: Murali Karicheri <[email protected]>
Cc: Lorenzo Pieralisi <[email protected]>
Cc: Bjorn Helgaas <[email protected]>