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>