powerpc/fsl: Fix SPI compatible on t208xrdb and t1040rdb

On the t208xrdb and t1040rdb, the SPI device is n25q512ax3
instead of n25q512a.

Signed-off-by: Hou Zhiqiang <Zhiqiang.Hou@freescale.com>
Signed-off-by: Scott Wood <oss@buserror.net>