mtd: nand: add more comment for MTD_NANDFLASH/MTD_MLCNANDFLASH

In current code, the MTD_NANDFLASH is used to represent both the SLC and
MLC. It is confusing to us.

By adding an explicit comment about these two macros, this patch makes it
clear that:
	MTD_NANDFLASH    : stands for SLC NAND,
	MTD_MLCNANDFLASH : stands for MLC NAND (including TLC).
Signed-off-by: Huang Shijie <b32955@freescale.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

include/uapi/mtd/mtd-abi.h

@@ -94,10 +94,10 @@ struct mtd_write_req {
 #define MTD_RAM			1
 #define MTD_ROM			2
 #define MTD_NORFLASH		3
-#define MTD_NANDFLASH		4
+#define MTD_NANDFLASH		4	/* SLC NAND */
 #define MTD_DATAFLASH		6
 #define MTD_UBIVOLUME		7
-#define MTD_MLCNANDFLASH	8
+#define MTD_MLCNANDFLASH	8	/* MLC NAND (including TLC) */
 
 #define MTD_WRITEABLE		0x400	/* Device is writeable */
 #define MTD_BIT_WRITEABLE	0x800	/* Single bits can be flipped */