mtd: rawnand: Move drivers for Ingenic SoCs to subfolder

Before adding support for more SoCs and seeing the number of files for
these drivers grow, we move them to their own subfolder to keep it tidy.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>