mtd: rawnand: ams-delta: Support custom driver initialisation

In preparation for extending the driver with custom I/O support, try to
obtain device specific initialisation routine from a matching device
table entry and run it as an additional step of device probe.

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-12-jmkrzyszt@gmail.com