dt-binding: arm/stm32: Add emtrion hardware emSBC-Argon

This patch presents the yaml patch for the emtrion GmbH
Argon board series.

Signed-off-by: Reinhold Mueller <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Alexandre Torgue <[e-mail]>

Documentation/devicetree/bindings/arm/stm32/stm32.yaml

···
           - enum:
               - st,stm32mp153
               - st,stm32mp157
+
+      - description: emtrion STM32MP1 Argon based Boards
+        items:
+          - const: emtrion,stm32mp157c-emsbc-argon
+          - const: emtrion,stm32mp157c-emstamp-argon
+          - const: st,stm32mp157
       - items:
           - enum:
               - st,stm32f429i-disco