mtd: nand: pxa3xx: enable NAND controller if the SoC needs it

Marvell recent SoCs like A7k/A8k do not boot with NAND flash
controller activated by default. Enabling the controller is a matter of
writing in a system controller register that may also be used for other
NAND related choices.

This change is needed to stay bootloader independent.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>