dm: pci: Add a function to get the controller for a bus

A PCI bus may be a bridge device where the controller is the bridge's
parent. Add a function to return the controller device, given a PCI device.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>