mtd: atmel_nand: Use of_device_get_match_data

Remove the need for forward declaration and the risk for a null pointer
when accessing the private part of the compatible match table, by using
the newly introduced of_device_get_match_data function.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>