mmc: core: Add ->hs400_prepare_ddr() callback

Some eMMC controllers need specific settings for HS400 mode before the
speed mode can be switched to DDR mode, during the HS400 initialization
sequence. For that reason, let's introduce a new host callback,
->hs400_prepare_ddr() and invoked it just before switching to DDR mode.

Signed-off-by: Yinbo Zhu <yinbo.zhu@nxp.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>