[POWERPC] Fix mpc52xx fdt to use correct device_type for sound devices

This corrects the documented interface for mpc52xx device trees.
Sound devices should be using 'sound' for the device_type field, not
the type of sound interface.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Sylvain Munaut <tnt@246tNt.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>