mmc: sdhci-esdhc-imx: enable hw auto retuning for STD_TUNING

Enable HW auto retuning when set SDHCI_CTRL_EXEC_TUNING and clear it
when clear SDHCI_CTRL_TUNED_CLK.

Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>