mtd: rawnand: ams-delta: Make the driver custom I/O ready

In order to be merged with "gpio-nand", the driver must support custom
(non-GPIO) I/O accessors.

Allow platforms to omit data GPIO port as well as NWE pin info from
device setup.  For the driver to still work on such platform, custom
I/O accessors as well as a custom probe function which initialises the
driver private structure with those accessors must be added to the
driver.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-14-jmkrzyszt@gmail.com