mmc: dw_mmc: add support tuning scheme

For the speed modes HS200 and SDR104, tuning is needed to determine the
correct sampling point. Actual tuning procedure is provided by specific
host controller driver.  This patch defines the tuning command and
tuning data.  Additionally, 'struct dw_mci_slot' is moved to header
file to consider the extensive usages in driver.

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>