mmc: host: Adds support for eMMC 4.5 HS200 mode

This patch adds support for the HS200 mode on the host side.
Also enables the tuning feature required when the HS200 mode
is selected.

Signed-off-by: Girish K S <girish.shivananjappa@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>