arm: kirkwood: dreamplug: use Device Tree to probe SDIO

Now that the mvsdio driver has a Device Tree binding, and the SDIO
controller is declared in kirkwood.dtsi, migrate the dreamplug board
to use the Device Tree to probe the SDIO controller and to mux this
interface properly.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Jason Cooper <jason@lakedaemon.net>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>