arm: kirkwood: mplcec4: 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 mplcec4 board to
use the Device Tree to probe the SDIO controller and to mux the pins
of the SDIO interface correctly.

This patch has not been tested, it remains to be tested by a person
having access to the hardware.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Stefan Peter <s.peter@mpl.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>