ASoC: core - Make platform probe more like codec probe.

In preparation for ASoC dynamic PCM support (AKA ASoC DSP)

Platform will also support DAPM so separate out the probe function
to simplify the code (just like the codec probe).

Signed-off-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>