mmc: sdhci: add __sdhci_execute_tuning() to header

Expose __sdhci_execute_tuning() so that it can be called from the
mmc host controller drivers.

In the sdhci-of-dwcmshc driver, sdhci_dwcmshc_th1520_ops sets
platform_execute_tuning to th1520_execute_tuning(). That function has
to manipulate phy registers before tuning can be performed. To avoid
copying the code verbatim from __sdhci_execute_tuning() into
th1520_execute_tuning(), make it possible for __sdhci_execute_tuning()
to be called from sdhci-of-dwcmshc.

Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>
Link: https://lore.kernel.org/r/20231109-th1520-mmc-v5-2-018bd039cf17@baylibre.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>