dt-bindings: mtd: Add SEAMA partition bindings

This types of NAND partitions appear in OpenWrt and
U-Boot.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230713-seama-partitions-v4-1-69e577453d40@linaro.org