ARM: dts: bcm2835_audio missing firmware reference

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: https://github.com/raspberrypi/linux/issues/4634
     https://github.com/raspberrypi/linux/issues/4635

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