ARM: dts: imx51-zii-rdu1: cleanup eMMC node

On RDU1, sdhc1 is used for eMMC, and that is 3.3V only.

Thus configure device node not to probe it as SD/SDIO and not try 1.8V.

Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>