mtd: rawnand: ams-delta: Make read pulses optional

Allow platforms to omit NRE pin from device configuration by requesting
that pin as optional.  In that case, also don't apply read pulse width
from chip SDR timings.  There should be no need for further code
adjustments as gpiolib can handle NULL GPIO descriptor pointers.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-9-jmkrzyszt@gmail.com