mmc: sdhci-xenon: ignore timing DDR52 in tuning

Emmc DDR52 mode uses fixed delay, return earlier if
timing is MMC_TIMING_MMC_DDR52 in execute tuning function.

Signed-off-by: Zhoujie Wu <zjwu@marvell.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>