dt-bindings: mtd: renesas: Describe Renesas R-Car Gen3 & RZ/N1 NAND controller

Add a Yaml description for this Renesas NAND controller.

As this controller is embedded on different SoC families, provide:
* a family-specific "r-car-gen3" compatible and a more specific
  "r8a77951" one
* a family-specific "rzn1" compatible and a more specific "r9a06g032"
  one

More compatibles can be added later if new SoCs with this controller
must be supported.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/linux-mtd/20211217142033.353599-2-miquel.raynal@bootlin.com