dt-bindings: mtd: mtk-nand: Update properties description

Update ecc step size, ecc strength, and parity bits supported on
each MTK NAND controller.

Signed-off-by: Xiaolei Li <[e-mail]>
Signed-off-by: Boris Brezillon <[e-mail]>

Documentation/devicetree/bindings/mtd/mtk-nand.txt

···
 - nand-on-flash-bbt:	Store BBT on NAND Flash.
 - nand-ecc-mode:	the NAND ecc mode (check driver for supported modes)
 - nand-ecc-step-size:	Number of data bytes covered by a single ECC step.
-			valid values: 512 and 1024.
+			valid values:
+			512 and 1024 on mt2701 and mt2712.
+			512 only on mt7622.
 			1024 is recommended for large page NANDs.
 - nand-ecc-strength:	Number of bits to correct per ECC step.
-			The valid values that the controller supports are: 4, 6,
-			8, 10, 12, 14, 16, 18, 20, 22, 24, 28, 32, 36, 40, 44,
-			48, 52, 56, 60.
+			The valid values that each controller supports:
+			mt2701: 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 28,
+				32, 36, 40, 44, 48, 52, 56, 60.
+			mt2712: 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 28,
+				32, 36, 40, 44, 48, 52, 56, 60, 68, 72, 80.
+			mt7622: 4, 6, 8, 10, 12, 14, 16.
 			The strength should be calculated as follows:
-			E = (S - F) * 8 / 14
+			E = (S - F) * 8 / B
 			S = O / (P / Q)
 				E :	nand-ecc-strength.
 				S :	spare size per sector.
···
 				O :	oob size.
 				P :	page size.
 				Q :	nand-ecc-step-size.
+				B :	number of parity bits needed to correct
+					1 bitflip.
+					According to MTK NAND controller design,
+					this number depends on max ecc step size
+					that MTK NAND controller supports.
+					If max ecc step size supported is 1024,
+					then it should be always 14. And if max
+					ecc step size is 512, then it should be
+					always 13.
 			If the result does not match any one of the listed
 			choices above, please select the smaller valid value from
 			the list.