mtd: Define MLC Flash as a different flash type

MLC NAND Flash has a different cell structure and differs in
functioning than the SLC NAND Flash. Hence we are considering it as
a different Flash type.

Signed-off-by: Rohit H.S <rohit.hs@samsung.com>
Signed-off-by: Raghav Gupta <gupta.raghav@samsung.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>