firmware: raspberrypi: Introduce vl805 init routine

The Raspberry Pi 4 gets its USB functionality from VL805, a PCIe chip
that implements xHCI. After a PCI reset, VL805's firmware may either be
loaded directly from an EEPROM or, if not present, by the SoC's
co-processor, VideoCore. RPi4's VideoCore OS contains both the non public
firmware load logic and the VL805 firmware blob. The function this patch
introduces triggers the aforementioned process.

Link: https://lore.kernel.org/r/20200505161318.26200-3-nsaenzjulienne@suse.de
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>