mmc: sdhci-esdhc-imx: enable SDR50 tuning for imx6q/dl

The imx6q/dl supports SDR50 tunning, enable it for a better timing
on SDR50 mode.

Signed-off-by: Dong Aisheng <b29396@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>