dt-bindings: dsp: fsl: Update binding document for remote proc driver

As there are two drivers for DSP on i.MX, one is for sound open
firmware, another is for remote processor framework. In order to
distinguish two kinds of driver, defining different compatible strings.

For remote proc driver, the properties firmware-name and fsl,dsp-ctrl
are needed and the mailbox channel is different with SOF.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1633944015-789-5-git-send-email-shengjiu.wang@nxp.com


[Fixed capital letter at the beginning of the subject line]
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>