mmc: core: Add host specific tuning support for eMMC HS400 mode

This adds a ->execute_hs400_tuning() host callback to enable optional
support for host specific tuning for eMMC HS400 mode. Additionally, share
mmc_get_ext_csd() through the public host headerfile, to allow it to be
used by the host drivers, which is needed to support the HS400 tuning.

Signed-off-by: Wenbin Mei <wenbin.mei@mediatek.com>
Link: https://lore.kernel.org/r/20210917124803.22871-3-wenbin.mei@mediatek.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>