ARM: dts: bcm2835: make the firmware node into a bus

This allows adding devices for which the firmware exposes control interface
via the mailbox. An example of such device is the GPIO expander.
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Eric Anholt <eric@anholt.net>

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

@@ -18,7 +18,9 @@ act {
 
 	soc {
 		firmware: firmware {
-			compatible = "raspberrypi,bcm2835-firmware";
+			compatible = "raspberrypi,bcm2835-firmware", "simple-bus";
+			#address-cells = <0>;
+			#size-cells = <0>;
 			mboxes = <&mailbox>;
 		};