FDT without audio

[Robert Swindells <rjs%fdy2.co.uk@localhost>]

I'm trying to convert a port over to use FDT and am getting an
error because it doesn't include any audio devices:

In file included from ../../../../dev/audio_dai.h:32:0,
                 from ../../../../dev/fdt/fdtvar.h:39,
                 from ../../../../arch/arm/fdt/arm_fdt.c:41:
../../../../dev/audio_if.h:49:2: error: #error "No 'audio* at audiobus?' or 'midi* at midibus?' or similar configured"
 #error "No 'audio* at audiobus?' or 'midi* at midibus?' or similar configured"
  ^~~~~
nbmkdep: compile failed.

*** Failed target:  arm_fdt.d