mtd: rawnand: ams-delta: Rename structures and functions to gpio_nand*

Another step in preparation for merging the driver with "gpio-nand".

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-15-jmkrzyszt@gmail.com