ARM: dts: imx6ull-colibri: enable UHS-I for USDHC1

Allows to use the SD interface at a higher speed mode if the card
supports it. For this the signaling voltage is switched from 3.3V to
1.8V under the usdhc1's drivers control.

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Reviewed-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>