warp: Select CONFIG_SYS_FSL_ESDHC_HAS_DDR_MODE

Warp has a DDR eMMC, so enable CONFIG_SYS_FSL_ESDHC_HAS_DDR_MODE for better
performance:

reading zImage
5790288 bytes read in 117 ms (47.2 MiB/s)

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>