mmc: dw_mmc: exynos: Support eMMC's HS400 mode

Implements HS400 mode support for exynos host driver.
This also include some updates as new mode is added.

Signed-off-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Alim Akhtar <alim.akhtar@samsung.com>
[Alim: addressed review comments]
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>