ASoC: wm_adsp: Make DSP preloader a supply widget

Currently the DSP loading is split into two widgets, the preloader that
is a snd_soc_dapm_dai_link widget which starts a thread to download
the firmware, and the DSP itself which is a snd_soc_dapm_out_drv and
synchronises the thread back in to the DAPM sequence. This allows the
firmware download to be overlapped with the rest of the path bring up.

The use of a snd_soc_dapm_dai_link widget requires the preloader to be part
of the audio path in DAPM, really a supply widget is a better fit for the
preloader. The preloader is something that needs to be done for the DSP to
function, not a part of the audio path itself.

This change makes the DSP preloader widget a supply widget, which as well
as probably being a better fit will also make it much simpler to power up
the preloader widget to trigger firmware download to the core independently
of the audio path coming up.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>