dt-bindings: arm: fsl: Add FRDM-IMX91S board

Add DT compatible string for NXP FRDM-IMX91S board

The FRDM-IMX91S is a low-cost, compact development board based on the
i.MX91 applications processor. It is a cost-optimized variant of the
FRDM-IMX91 board, with notable hardware differences requiring a separate
DTS:
- 512MB LPDDR4 (FRDM-IMX91 uses 1GB)
- 256MB FlexSPI-NAND (FRDM-IMX91 uses 8GB eMMC)
- Single GbE port (FRDM-IMX91 has dual GbE)
- PMIC PF9453 (FRDM-IMX91 uses PCA9451A)

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Yanan Yang <yanan.yang@nxp.com>
Signed-off-by: Frank Li <Frank.Li@nxp.com>

Author: yananyang-nxp

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -1432,6 +1432,7 @@ properties:
           - enum:
               - fsl,imx91-11x11-evk       # i.MX91 11x11 EVK Board
               - fsl,imx91-11x11-frdm      # FRDM i.MX91 Development Board
+              - fsl,imx91-11x11-frdm-s    # FRDM i.MX91S Development Board
           - const: fsl,imx91
 
       - description: i.MX93 based Boards