dt-bindings: stm32: document stm32mp257f-dk board

Add new entry for stm32mp257f-dk board.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20250225-b4-stm32mp2_new_dts-v2-1-1a628c1580c7@foss.st.com
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>

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

@@ -187,6 +187,7 @@ properties:
       - description: ST STM32MP257 based Boards
         items:
           - enum:
+              - st,stm32mp257f-dk
               - st,stm32mp257f-ev1
           - const: st,stm32mp257