mmc: sdhci: usdhc: do not do tuning for DDR50 mode.

DDR50 tuning is optinally defined in sd 3.0 spec. And i.MX
uSDHC internally already uses a fixed optimized timing for
DDR50, normally does not require tuning for DDR50 mode.

This patch specify a new execute_tuning function for i.MX
uSDHC, do not impact i.MX eSDHC.

Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>