uboot-envtools: add env settings for ubnt,unifi-6-lr-v3

uboot-envtools: add env settings for ubnt,unifi-6-lr-v3

Using the same configuration as my earlier Unifi 6-LRv2s:

```bash
$ cat /etc/fw_env.config
/dev/mtd3 0x0 0x1000 0x1000 1
$ fw_printenv
arch=arm
baudrate=115200
board=mt7622_evb
board_name=mt7622_evb
bootcmd=bootubnt
bootdelay=3
bootfile=uImage
cpu=armv7
device_model=U6-LR
ethact=mtk_eth
ethaddr=<redacted>
ipaddr=<redacted>
is_ble_stp=true
is_default=true
loadaddr=0x5007FF28
macaddr=<redacted>
serverip=<redacted>
soc=mt7622
stderr=serial
stdin=serial
stdout=serial
vendor=mediatek
```

Signed-off-by: Joel Low <joel@joelsplace.sg>
Link: https://github.com/openwrt/openwrt/pull/16127
Signed-off-by: Robert Marko <robimarko@gmail.com>