dm: imx: Move cm_fx6 to use driver model for serial and GPIO

Now that serial and GPIO are available for iMX.6, move cm_fx6 over as an
example.

Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>