mmc: Add support for wait_dat0 callback

There is no wait_dat0 mmc ops, causing operations waiting for data
line state change (e.g mmc_switch_voltage) to fallback to a 250ms
active delay. mmc_ops still used when DM_MMC is not enabled, which
is often the case for SPL. The result can be unexpectly long SPL
boot time.

This change adds support for wait_dat0() mmc operation.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/include/mmc.h b/include/mmc.h
index 9b4dc68..073b01f 100644
--- a/include/mmc.h
+++ b/include/mmc.h
@@ -561,6 +561,7 @@
 	int (*getwp)(struct mmc *mmc);
 	int (*host_power_cycle)(struct mmc *mmc);
 	int (*get_b_max)(struct mmc *mmc, void *dst, lbaint_t blkcnt);
+	int (*wait_dat0)(struct mmc *mmc, int state, int timeout_us);
 };
 
 static inline int mmc_hs400_prepare_ddr(struct mmc *mmc)