mmc: sdhc: add execute tuning callback

MSM SDHC doesn't support tuning specified by SDHC-3.0, hence
add a callback to facilitate custom tuning mechanism.

Change-Id: I9648638b3a55e2722acf44e18778439b77280d6d
Signed-off-by: Asutosh Das <asutoshd@codeaurora.org>