dt-bindings: reset: Add a binding for the RPi Firmware reset controller

The firmware running on the RPi VideoCore can be used to reset and
initialize HW controlled by the firmware.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Link: https://lore.kernel.org/r/20200629161845.6021-2-nsaenzjulienne@suse.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>