dt-bindings: remoteproc: imx_rproc: Support i.MX8MN/P MMIO

The MX8M CM7 boot via SMC call is problematic, since not all versions
of ATF support this interface. Document MMIO support used to boot the
CM7 on MX8MN/MP instead and discern MMIO interface using DT compatible
string. Document GPR register syscon phandle which is required by the
MMIO interface too.
Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230724222418.163220-1-marex@denx.deSigned-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>

Documentation/devicetree/bindings/remoteproc/fsl,imx-rproc.yaml

@@ -20,7 +20,9 @@ properties:
       - fsl,imx7ulp-cm4
       - fsl,imx8mm-cm4
       - fsl,imx8mn-cm7
+      - fsl,imx8mn-cm7-mmio
       - fsl,imx8mp-cm7
+      - fsl,imx8mp-cm7-mmio
       - fsl,imx8mq-cm4
       - fsl,imx8qm-cm4
       - fsl,imx8qxp-cm4
@@ -70,6 +72,11 @@ properties:
     description:
       Specify CPU entry address for SCU enabled processor.
 
+  fsl,iomuxc-gpr:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      Phandle to IOMUXC GPR block which provide access to CM7 CPUWAIT bit.
+
   fsl,resource-id:
     $ref: /schemas/types.yaml#/definitions/uint32
     description:
@@ -79,6 +86,19 @@ properties:
 required:
   - compatible
 
+allOf:
+  - if:
+      properties:
+        compatible:
+          not:
+            contains:
+              enum:
+                - fsl,imx8mn-cm7-mmio
+                - fsl,imx8mp-cm7-mmio
+    then:
+      properties:
+        fsl,iomuxc-gpr: false
+
 additionalProperties: false
 
 examples: