ARM: imx: Enable dts for i.MX6SX-SDB

Enable DTS and OF_CONTROL for i.MX6SX-SDB.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>