[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/drivers/mtd/mtdpart.c b/drivers/mtd/mtdpart.c
index 89692f8..bafd2fb 100644
--- a/drivers/mtd/mtdpart.c
+++ b/drivers/mtd/mtdpart.c
@@ -340,6 +340,7 @@
 		slave->mtd.oobsize = master->oobsize;
 		slave->mtd.ecctype = master->ecctype;
 		slave->mtd.eccsize = master->eccsize;
+		slave->mtd.subpage_sft = master->subpage_sft;
 
 		slave->mtd.name = parts[i].name;
 		slave->mtd.bank_size = master->bank_size;