dt-bindings: serial: samsung: include generic dtschema to match bluetooth child

Include the generic serial.yaml dtschema so the child node like
"bluetooth" will be properly matched:

  arch/arm/boot/dts/exynos4210-universal_c210.dt.yaml:
    serial@13800000: 'bluetooth' does not match any of the regexes: 'pinctrl-[0-9]+'
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20210409172027.11296-1-krzysztof.kozlowski@canonical.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/samsung_uart.yaml

@@ -79,9 +79,11 @@ required:
   - interrupts
   - reg
 
-additionalProperties: false
+unevaluatedProperties: false
 
 allOf:
+  - $ref: /schemas/serial.yaml#
+
   - if:
       properties:
         compatible: