mtd: rawnand: arasan: Leverage additional GPIO CS

mtd: rawnand: arasan: Leverage additional GPIO CS

Make use of the cs-gpios DT property as well as the core helper to parse
it so that the Arasan controller driver can now assert many more chips
than natively.

The Arasan controller has an internal limitation: RB0 is tied to CS0 and
RB1 is tied to CS1. Hence, it is possible to use external GPIOs as long
as one or the other native CS is not used (or configured to be driven as
a GPIO) and that all additional CS are physically wired on its
corresponding RB line. Eg. CS0 is used as a native CS, CS1 is not used
as native CS and may be used as a GPIO CS, CS2 is an additional GPIO
CS. Then the target asserted by CS0 should also be wired to RB0, while
the targets asserted by CS1 and CS2 should be wired to RB1.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210526093242.183847-5-miquel.raynal@bootlin.com