mtd: atmel_nand: introduce a new compatible string for sama5d4 chip

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

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>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>