mmc: dw_mmc: add switch_voltage

switch_voltage is required on some platform since special register accessing

Signed-off-by: Jorge A. Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Dan Yuan <yuandan@hisilicon.com>
Signed-off-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
diff --git a/drivers/mmc/host/dw_mmc.h b/drivers/mmc/host/dw_mmc.h
index f45ab91..c723617 100644
--- a/drivers/mmc/host/dw_mmc.h
+++ b/drivers/mmc/host/dw_mmc.h
@@ -287,5 +287,7 @@
 	int		(*execute_tuning)(struct dw_mci_slot *slot);
 	int		(*prepare_hs400_tuning)(struct dw_mci *host,
 						struct mmc_ios *ios);
+	int		(*switch_voltage)(struct mmc_host *mmc,
+					  struct mmc_ios *ios);
 };
 #endif /* _DW_MMC_H_ */