nand: atmel_nand: according to pmecc version to perform 0xff page correction

In SAMA5D4 chip, the PMECC ip can generate 0xff pmecc decode value for
all 0xff sector.
So for SAMA5D4, if pmecc find bit errors, just let PMECC hw to correct.

Signed-off-by: Josh Wu <josh.wu@atmel.com>