mtd: rawnand: ams-delta: Handle more GPIO pins as optional

In order to make the driver more useful on platforms other than Amstrad
Delta, allow GPIO descriptor pointers of possibly non-critical NWP and
NCE pins to be initialised as NULL.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-10-jmkrzyszt@gmail.com