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

As the PMECC hardware has different version. In SAMA5D4 chip, the PMECC ip
can generate 0xff pmecc ECC value for all 0xff sector.

According to this, add PMECC version check, if it's SAMA5D4 then we always
let PMECC hardware to correct it.

Signed-off-by: Josh Wu <josh.wu@atmel.com>
Acked-by: Bo Shen <voice.shen@atmel.com>
Acked-by: Andreas Bießmann <andreas.devel@googlemail.com>