mmc: sdhci: export sdhci_execute_tuning()

Some SDHCI-compat controllers support not only SD, but also eMMC,
but they use different commands for tuning: CMD19 for SD, CMD21 for
eMMC.

Due to the difference of the underlying mechanism, some controllers
(at least, the Cadence IP is the case) provide their own registers
for the eMMC tuning.

This commit will be useful when we want to override .execute_tuning
callback (for eMMC HS200 tuning), but still let it fall back to
sdhci_execute_tuning() for SD timing.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>