usb: xhci: Load Raspberry Pi 4 VL805's firmware

usb: xhci: Load Raspberry Pi 4 VL805's firmware

When needed, RPi4's co-processor (called VideoCore) has to be instructed
to load VL805's firmware (the chip providing xHCI support). VideCore's
firmware expects the board's PCIe bus to be already configured in order
for it to load the xHCI chip firmware. So we have to make sure this
happens in between the PCIe configuration and xHCI startup.

Introduce a callback in xhci_pci_probe() to run this platform specific
routine.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
[mszyprow: backport of https://patchwork.ozlabs.org/project/uboot/list/?series=174860&state=%2A&archive=both
 the final mainline patch depends on DTS changes not present in vendor tree]
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Change-Id: I1391806cae261d1ce22137e1a8baf5e261cb865a