Now on colibiri imx6ull, Wi-Fi module is interfaced to SoC via 3.3V SDIO on USCHC2. Is the voltage fixed to 3.3V of Wi-Fi module? Can I just modify device tree enabling 1.8V interface to support SD3.0 UHS-I, iMX6ULL SoC and WM828CC6 do support SDIO 3.0? Since new iMX6ULL will come, will UHS-I be used by Wi-Fi module? Thanks!
No, there is no option to switch interface voltage for the wifi SDIO on imx6ull. This will not change, WiFi on imx6ull will work only in SDIO high-speed mode.