dt-bindings: mtd: st,stm32: add MP25 support

Add a compatible string to support MP25 SoC.

FMC2 IP supports up to 4 chip select. On MP1 SoC, only 2 of them are
available when on MP25 SoC, the 4 chip select are available.

Signed-off-by: Christophe Kerello <christophe.kerello@foss.st.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20240219140505.85794-2-christophe.kerello@foss.st.com