dt-bindings: samsung: exynos-usi: Restrict possible samsung,mode values

"samsung,mode" property defines the desired mode of the serial engine
(e.g. I2C or SPI) and only few values are allowed/used by Linux driver.

Cc: Ivaylo Ivanov <ivo.ivanov.ivanov1@gmail.com>
Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20250103082549.19419-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>

Documentation/devicetree/bindings/soc/samsung/exynos-usi.yaml

@@ -64,6 +64,7 @@ properties:
 
   samsung,mode:
     $ref: /schemas/types.yaml#/definitions/uint32
+    enum: [0, 1, 2, 3]
     description:
       Selects USI function (which serial protocol to use). Refer to
       <include/dt-bindings/soc/samsung,exynos-usi.h> for valid USI mode values.