ARM: dts: imx6qdl-microsom.dtsi: avoid boot-time UHS warning

Avoid the following warning:

sdhci-esdhc-imx 2190000.usdhc: could not get ultra high speed state, work on normal mode

which occurs regularly at boot each time the SDHCI interface for the
Broadcom WiFi is probed at boot.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>