The i.MX6 supports 1.8v/3.3v eMMC DDR mode, so add this flag.
Signed-off-by: NDong Aisheng <b29396@freescale.com>
Acked-by: NShawn Guo <shawn.guo@linaro.org>
Signed-off-by: NChris Ball <cjb@laptop.org>