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>