mmc: Add a execute_tuning() callback to the mmc operations.

Tuning is a mandatory step in the initialization of SDR104 and HS200 modes.
This callback execute the tuning process.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/drivers/mmc/mmc.c b/drivers/mmc/mmc.c
index be68d8d..b06e4bc 100644
--- a/drivers/mmc/mmc.c
+++ b/drivers/mmc/mmc.c
@@ -1199,6 +1199,11 @@
 }
 
 #if !CONFIG_IS_ENABLED(DM_MMC)
+static int mmc_execute_tuning(struct mmc *mmc, uint opcode)
+{
+	return -ENOTSUPP;
+}
+
 static void mmc_send_init_stream(struct mmc *mmc)
 {
 }