mx6_common: Do not select esdhc DDR mode for all boards

CONFIG_SYS_FSL_ESDHC_HAS_DDR_MODE should be selected only by boards that really
have a DDR-capable eMMC, so remove this option from common code to avoid
regressions.

Reported-by: Stefan Roese <sr@denx.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>
Reviewed-by: Volodymyr Riazantsev <volodymyr.riazantsev@globallogic.com>
Tested-by: Stefan Roese <sr@denx.de>