ARM: dts: colibri-imx6ull: limit SDIO clock to 25MHz

NXP and AzureWave don't recommend using SDIO bus mode 3.3V@50MHz due
to noise affecting the wireless throughput. Colibri iMX6ULL uses only
3.3V signaling for Wi-Fi module AW-CM276NF.

Limit the SDIO Clock on Colibri iMX6ULL to 25MHz.

Fixes: c2e4987e ("ARM: dts: imx6ull: add Toradex Colibri iMX6ULL support")
Signed-off-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>