ARM: dts: bcm2835_audio missing firmware reference

The firmware driver has been changed to count its clients. An earlier
commit removed the downstream patch permitting the hacky technique of
passing NULL to rpi_firmware_get to pick up the last instace, but
failed to add the necessary "firmware" property to the bcm2835_audio
node. Correct that omission.

See: #4634
     #4635

Signed-off-by: Phil Elwell <phil@raspberrypi.com>

arch/arm/boot/dts/bcm270x-rpi.dtsi

@@ -148,6 +148,7 @@
 	/* Onboard audio */
 	audio: bcm2835_audio {
 		compatible = "brcm,bcm2835-audio";
+		brcm,firmware = <&firmware>;
 		brcm,pwm-channels = <8>;
 		status = "disabled";
 	};