imx: mx7: fix build warning when CONFIG_IMX_RDC not enabled

Fix build warning when CONFIG_IMX_RDC not defined in defconfig.

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