mtd: atmel_nand: Support PMECC on SAMA5D2

Starting with the SAMA5D2, there is a new revision of the Atmel PMECC
controller that can correct 32 bits in each sector. This controller is
not 100% compatible with the previous revision that corrected a maximum
of 24 bits by sector, as some register addresses overlap.

Using information from the device tree, we can configure the driver to
work with both versions.

For the binding:
Acked-by: Rob Herring <robh@kernel.org>

Tested-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>