Add OpenWrt support for Xiaomi "Redmi AX6000"

Yes here's my findings. I'll compare them to my RT3200 again.

AX6000 5GHz, 80MHz channels: Performance on Apple devices is horrible (6-12mbps at -68 RSSI, one wall between device and AP). Other devices speeds are great, mostly 600-700mbps, even had a RT3200 hit over 1gbps.

AX6000 5GHz, 160MHz channels: Performance on Apple devices appears normal, 200-300mbps iperf3 tests. 160MHz does appear to nerf my other E8450 in Client WDS mode (stuck at ~144mbps, almost like it locks to 802.11n).

This also doesn't make sense because Apple devices don't support 160MHz so I don't really understand how the channel bandwidth "fixes" that part. I tested both channel 36 and 161 in 80MHz channel width, both tests showed 6-12mbps on Apple devices so channel is likely ruled out. I'll reiterate this part as I mentioned in the other thread, seems like Apple devices get stuck in low power mode when their RSSI goes past RSSI somewhere past -60 ish.