mmc: add a mmc_hs400_prepare_ddr() interface

Add a mmc_hs400_prepare_ddr() interface for controllers
which needs preparation before switching to DDR mode for
HS400 mode.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
diff --git a/drivers/mmc/mmc-uclass.c b/drivers/mmc/mmc-uclass.c
index b90e1ee..ec59bcd 100644
--- a/drivers/mmc/mmc-uclass.c
+++ b/drivers/mmc/mmc-uclass.c
@@ -142,6 +142,21 @@
 }
 #endif
 
+int dm_mmc_hs400_prepare_ddr(struct udevice *dev)
+{
+	struct dm_mmc_ops *ops = mmc_get_ops(dev);
+
+	if (ops->hs400_prepare_ddr)
+		return ops->hs400_prepare_ddr(dev);
+
+	return 0;
+}
+
+int mmc_hs400_prepare_ddr(struct mmc *mmc)
+{
+	return dm_mmc_hs400_prepare_ddr(mmc->dev);
+}
+
 int dm_mmc_host_power_cycle(struct udevice *dev)
 {
 	struct dm_mmc_ops *ops = mmc_get_ops(dev);