dt-bindings: arm: fsl: Add FRDM-IMX91 board

Add the board FRDM-IMX91 in the binding document.

Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Reviewed-by: Francesco Valla <francesco@valla.it>
Tested-by: Francesco Valla <francesco@valla.it>
Signed-off-by: Joseph Guo <qijian.guo@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Author: Joseph-Guo-nxp

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -1415,6 +1415,7 @@ properties:
         items:
           - enum:
               - fsl,imx91-11x11-evk       # i.MX91 11x11 EVK Board
+              - fsl,imx91-11x11-frdm      # FRDM i.MX91 Development Board
           - const: fsl,imx91
 
       - description: i.MX93 based Boards