mmc: dw_mmc: add the capability to support hs200 mode

As host controller can support eMMC's HS200 mode at 1.8V or 1.2V,
these capability will be added.

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>