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: 7db2f2df ("dt-bindings: dsp: fsl: Add DSP core binding support")
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>