dt-bindings: mtd: Drop unneeded quotes

Cleanup bindings dropping unneeded quotes. Once all these are fixed,
checking for this can be enabled in yamllint.

Signed-off-by: Rob Herring <robh@kernel.org>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de> #rockchip
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230317233631.3968509-1-robh@kernel.org