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/20200821035420.380495-16-robh@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Murali Karicheri <m-karicheri2@ti.com>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>