mtd: rawnand: ams-delta: Push inversion handling to gpiolib

Let platforms take care of declaring correct GPIO pin polarity so we
can just ask a GPIO line to be asserted or deasserted and gpiolib deals
with the rest depending on how the platform is configured.

Inspired by similar changes to regulator drivers by Linus Walleij
<linus.walleij@linaro.org>, thanks!

Signed-off-by: Janusz Krzysztofik <jmkrzyszt@gmail.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200212003929.6682-7-jmkrzyszt@gmail.com