dt-bindings: mtd: oxnas-nand: remove obsolete bindings

Due to lack of maintenance and stall of development for a few years now,
and since no new features will ever be added upstream, remove the
for OX810 and OX820 nand bindings.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230630-topic-oxnas-upstream-remove-v2-6-fb6ab3dea87c@linaro.org