mtd: nand: pxa3xx: Add supported ECC strength and step size to the DT binding

This commit updates the devicetree binding documentation for this driver
with the supported ECC strength and step size combinations.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

Documentation/devicetree/bindings/mtd/pxa3xx-nand.txt

@@ -17,6 +17,14 @@ Optional properties:
  - num-cs:			Number of chipselect lines to usw
  - nand-on-flash-bbt: 		boolean to enable on flash bbt option if
 				not present false
+ - nand-ecc-strength:           number of bits to correct per ECC step
+ - nand-ecc-step-size:          number of data bytes covered by a single ECC step
+
+The following ECC strength and step size are currently supported:
+
+ - nand-ecc-strength = <1>, nand-ecc-step-size = <512>
+ - nand-ecc-strength = <4>, nand-ecc-step-size = <512>
+ - nand-ecc-strength = <8>, nand-ecc-step-size = <512>
 
 Example: