ARM: dts: imx6: RDU2: detail USDHC description

None of the slots support 1.8V signaling or SDIO. There is no point in
probing for a SD card on the eMMC controller.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>