pci: brcmstb: Cleanup controller state before handover

pci: brcmstb: Cleanup controller state before handover

Make sure we handover the PCIe controller in a clean state. Some of the
devices hanging from the PCIe bus might need to be properly reset
through #PERST in order for Linux to be able to initialize them.

This is specially important in order to properly initialize Raspberry Pi
4 B and 400's USB chip.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>