mmc: rework selection of bus speed mode

mmc: rework selection of bus speed mode

Current implementation for bus speed mode selection is too
complicated. This patch is to simplify the codes and remove
some duplicate parts.

The following changes are including:
* Adds functions for each mode selection(HS, HS-DDR, HS200 and etc)
* Rearranged the mode selection sequence with supported device type
* Adds maximum speed for HS200 mode(hs200_max_dtr)
* Adds field definition for HS_TIMING of EXT_CSD

Signed-off-by: Seungwon Jeon <[email protected]>
Tested-by: Jaehoon Chung <[email protected]>
Acked-by: Jaehoon Chung <[email protected]>
Signed-off-by: Ulf Hansson <[email protected]>
Signed-off-by: Chris Ball <[email protected]>