mtd: nand: gpio: make nCE GPIO optional

On some hardware, the nCE signal is wired to the ChipSelect associated
to bus address of the NAND, so it is automatically driven during the
memory access and it is not managed by a GPIO.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>