mtd: rawnand: ingenic: Separate top-level and SoC specific code

The ingenic-nand driver uses an API provided by the jz4780-bch driver.
This makes it difficult to support other SoCs in the jz4780-bch driver.
To work around this, we separate the API functions from the SoC-specific
code, so that these API functions are SoC-agnostic.

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