ARM: dts: imx6dl-colibri: Add usdhc1 sleep pin configuration

The Toradex board Iris V2 has a SD-card slot with switchable
power.
Add a pinctrl sleep used when the card power is off to avoid
backfeeding to the card and add the "sleep" pinctrl to the
usdhc1 controller.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>