ARM: dts: bcm283x: Fix vc4's firmware bus DMA limitations

The bus is virtual and devices have to inherit their DMA constraints
from the underlying interconnect. So add an empty dma-ranges property to
the bus node, implying the firmware bus' DMA constraints are identical to
its parent's.

Fixes: 7dbe8c6 ("ARM: dts: Add minimal Raspberry Pi 4 support")
Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

arch/arm/boot/dts/bcm2835-rpi.dtsi

@@ -15,6 +15,7 @@
 		firmware: firmware {
 			compatible = "raspberrypi,bcm2835-firmware", "simple-bus";
 			mboxes = <&mailbox>;
+			dma-ranges;
 		};
 
 		power: power {