mtd: atmel_nand: pmecc: add new dt property that support erased page correction

Since in SAMA5D4 chip, the PMECC can correct bit flips in erased page.
So we add a DT property to indicate this hardware charactor.

If the PMECC support correct bitflip erased page (all data are 0xff).
Then we can use the PMECC correct the page and skip the erased page
check.

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