MMC: add erase function to both mmc and sd

Erase is a very basic function since the begin of sd specification is
announced. Although we could write a bulk of full 0xff memory to the
range to take place of erase, it is more convenient and safe to
implement the erase function itself.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/include/part.h b/include/part.h
index 3cdae02..5243511 100644
--- a/include/part.h
+++ b/include/part.h
@@ -49,6 +49,9 @@
 				       unsigned long start,
 				       lbaint_t blkcnt,
 				       const void *buffer);
+	unsigned long   (*block_erase)(int dev,
+				       unsigned long start,
+				       lbaint_t blkcnt);
 	void		*priv;		/* driver private struct pointer */
 }block_dev_desc_t;