imx: serial_mxc: use CONFIG_IS_ENABLED instead of ifdef

Kconfig allows boards to configured with DM_SERIAL and still
have SPL_DM_SERIAL disabled.  This patch changes the ifdef's
to CONFIG_IS_ENABLED to allow the modes to differ between
SPL and U-Boot.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed