Enable the exposed SD card slots in the DT of the r8a7796/salvator-x.

Based on work for the r8a7795/salvator-x by Ai Kyuse.
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>