dt-bindings: arm: fsl: Add iMX8MN BSH SMM S2 boards

Add bindings for BSH SystemMaster (SMM) S2 board family, which consists
of: iMX8MN SMM S2 and iMX8MN SMM S2 PRO boards.
Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -807,6 +807,8 @@ properties:
         items:
           - enum:
               - beacon,imx8mn-beacon-kit  # i.MX8MN Beacon Development Kit
+              - bsh,imx8mn-bsh-smm-s2     # i.MX8MN BSH SystemMaster S2
+              - bsh,imx8mn-bsh-smm-s2pro  # i.MX8MN BSH SystemMaster S2 PRO
               - fsl,imx8mn-ddr4-evk       # i.MX8MN DDR4 EVK Board
               - fsl,imx8mn-evk            # i.MX8MN LPDDR4 EVK Board
               - gw,imx8mn-gw7902          # i.MX8MM Gateworks Board