dt-bindings: arm: imx: add Protonic PRT8MM board compatible

Add the compatible string for the Protonic PRT8MM board, which is a
reference design for a low-cost agricultural terminal.
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -771,6 +771,7 @@ properties:
               - gw,imx8mm-gw7902          # i.MX8MM Gateworks Board
               - kontron,imx8mm-n801x-som  # i.MX8MM Kontron SL (N801X) SOM
               - variscite,var-som-mx8mm   # i.MX8MM Variscite VAR-SOM-MX8MM module
+              - prt,prt8mm                # i.MX8MM Protonic PRT8MM Board
           - const: fsl,imx8mm
 
       - description: Engicam i.Core MX8M Mini SoM based boards