dt-bindings: dsp: Fix fsl,dsp example

The fsl,dsp binding requires a memory-region, yet its example doesn't have
one which results in a warning. Let's add a memory-region phandle to the
example.

Fixes: 7db2f2dfc701 ("dt-bindings: dsp: fsl: Add DSP core binding support")
Signed-off-by: Maxime Ripard <[e-mail]>
Reviewed-by: Daniel Baluta <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/dsp/fsl,dsp.yaml

···
                         <&pd IMX_SC_R_DSP_RAM>;
         mbox-names = "txdb0", "txdb1", "rxdb0", "rxdb1";
         mboxes = <&lsio_mu13 2 0>, <&lsio_mu13 2 1>, <&lsio_mu13 3 0>, <&lsio_mu13 3 1>;
+        memory-region = <&dsp_reserved>;
     };