PCI: dwc: Save root bus for driver remove hooks

Currently DWC host does not support the remove callback, but nothing
prevents us from supporting it.

Save the root bus for clean up work in driver remove code paths to allow
DWC host drivers to implement their remove hook as, eg:

  static int foo_pcie_remove(struct platform_device *pdev)
  {
    ...
    pci_stop_root_bus(pp->root_bus);
    pci_remove_root_bus(pp->root_bus);
    dw_pcie_free_msi(pp);
    ...
  }

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>