ASoC: au1x: update db1200 machine to the new way of things

The use of the "soc-audio" platform device is no longer en vogue,
update the code to the newer, simpler way of doing things.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>