arm: mvebu: dts: turris_mox: update sdhci properties

With recent changes to the mmc subsystem (chip detect code etc) update
the sdhci node of the Turris Mox device tree.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>