BCM270X_DT: Fix rpi-dac overlay

The rpi-dac overlay is almost identical to the hifiberry-dac overlay -
the codec used is different but it also doesn't sit on the I2C bus. As
a result, when the overlays were modified for dynamic loading and it
was discovered that the hifiberry-dac overlay didn't work any more, the
the rpi-dac overlay was also broken.

The failure was caused by the fact that outside a bus, device names are
constructed from the concatenation of the path elements leading to it,
so moving the codec instantiation inside /soc caused the device name
to get a "soc" added, breaking ALSA's naming.

See: http://forum.kodi.tv/showthread.php?tid=269814&pid=2349776#pid2349776



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