dt-bindings: arm: qcom: Document xiaomi,natrium board

Document Xiaomi Mi 5s Plus (xiaomi-natrium) smartphone which is based on
Snapdragon 821 SoC.
Signed-off-by: Alec Su <ae40515@yahoo.com.tw>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220606024706.22861-2-ae40515@yahoo.com.tw

Documentation/devicetree/bindings/arm/qcom.yaml

@@ -196,6 +196,7 @@ properties:
               - sony,kagura-row
               - sony,keyaki-row
               - xiaomi,gemini
+              - xiaomi,natrium
               - xiaomi,scorpio
           - const: qcom,msm8996