ASoC: SOF: imx8m: Fix DSP control regmap retrieval

According to Documentation/devicetree/bindings/dsp/fsl,dsp.yaml
fsl,dsp-ctrl is a phandle to syscon block so we need to use correct
function to retrieve it.

Currently there is no SOF DSP DTS merged into mainline so there is no
need to support the old way of retrieving the dsp control node.

Fixes: 9ba23717

 ("ASoC: SOF: imx8m: Implement DSP start")
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://patch.msgid.link/20240715151653.114751-1-daniel.baluta@oss.nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>