mmc: add a hs400_tuning flag

Some controllers may have difference between HS200 tuning
and HS400 tuning, such as different registers setting,
different procedure, or different errata.

This patch is to add a hs400_tuning flag to identify the
tuning for HS400 mode.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>