dm: mmc: Support erase

At present erase is not suported with CONFIG_DM_OPS. Add it so that MMC
devices can be erased.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/mmc/mmc-uclass.c b/drivers/mmc/mmc-uclass.c
index 425abb1..77424cd 100644
--- a/drivers/mmc/mmc-uclass.c
+++ b/drivers/mmc/mmc-uclass.c
@@ -262,6 +262,7 @@
 	.read	= mmc_bread,
 #ifndef CONFIG_SPL_BUILD
 	.write	= mmc_bwrite,
+	.erase	= mmc_berase,
 #endif
 	.select_hwpart	= mmc_select_hwpart,
 };