dt-bindings: mtd: renesas: Fix the NAND controller description

Add the missing power-domain property which is needed on all the
RZ/N1 SoC IPs.

Suggested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/linux-mtd/20220513104957.257721-2-miquel.raynal@bootlin.com