mtd: rawnand: ams-delta: Use struct gpio_nand_platdata

In order to be able to move the hardcoded Amstrad Delta partition info
from the driver code to the board file, reuse gpio_nand_platdata
structure owned by "gpio-nand" driver and try to obtain information
on device partitions from device platform data.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-3-jmkrzyszt@gmail.com