mmc: dw_mmc: exynos: add variable delay tuning sequence

Implements variable delay tuning. In this change, exynos host can
determine the correct sampling point for the HS200 and SDR104 speed mode.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Tested-by: Alim Akhtar <alim.akhtar@samsung.com>
Signed-off-by: Chris Ball <cjb@laptop.org>