[MTD] NAND: add subpage write support

Many SLC NANDs support up to 4 writes at one NAND page. Add support
of this feature.

Signed-off-by: Artem Bityutskiy <dedekind@infradead.org>
diff --git a/include/linux/mtd/mtd.h b/include/linux/mtd/mtd.h
index e34bbc9..18acb6d 100644
--- a/include/linux/mtd/mtd.h
+++ b/include/linux/mtd/mtd.h
@@ -200,6 +200,8 @@
 
 	/* ECC status information */
 	struct mtd_ecc_stats ecc_stats;
+	/* Subpage shift (NAND) */
+	int subpage_sft;
 
 	void *priv;