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