BCM270X_DT: Sound DT adjustments for Dynamic DT

There are a number of issues when loading and unloading overlays for
sound cards:
1) The "sound" and "audio" nodes must be children of a bus node, such
   as "/soc", otherwise the DT changes don't result in platform devices
   being created and deleted.
2) The "/sound" node must have a "disabled" status property, otherwise
   setting the status to "okay" won't be detected.
3) ALSA doesn't like having components unloaded under its feet, and it
   is easy to deadlock or crash. Ordering the overlay fragments so that
   the sound card appears last avoids this problem.

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