ARM: dts: Move audio node under the vchiq parent

VCHIQ kernel clients are now instantiated as platform drivers rather
than using DT, but the children of the vchiq device can optionally be
given a sub-node of the vchiq parent for configuration and to disable
them.

Move the existing audio node beneath the vchiq parent, to prevent
multiple instantiation and unpleasant warnings. Note that the node
name has to match the module name - "bcm2835_audio".

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