arm: rpi: Add function to trigger VL805's firmware load

On the Raspberry Pi 4, after a PCI reset, VL805's (a xHCI chip) firmware
may either be loaded directly from an EEPROM or, if not present, by the
SoC's VideCore (the SoC's co-processor). Introduce the function that
informs VideCore that VL805 may need its firmware loaded.

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