xiaomi-natrium: new device

Add support for Xiaomi Mi 5s Plus.

Most are based on xiaomi-scorpio and xiaomi-gemini.

https://wiki.postmarketos.org/wiki/Xiaomi_Mi_5s_Plus_(xiaomi-natrium)

device/testing/device-xiaomi-natrium/30-gpu-firmware.files

+./lib/firmware/qcom/a530_pfp.fw
+./lib/firmware/qcom/a530_pm4.fw
+./lib/firmware/qcom/a530v3_gpmu.fw2
+./lib/firmware/qcom/msm8996/natrium/a530_zap.mbn