mx6cuboxi: enable OF_PLATDATA

As both MMC and GPIO driver now supports OF_PLATDATA, enable it in
defconfig in order to reduce the SPL footprint. After applying this
setting the SPL reduction is 5 KB, which partially compensates the
increment due to DM.

Signed-off-by: Walter Lozano <walter.lozano@collabora.com>
Reviewed-by: Simon Glass <sjg@chromium.org>