spi: dt-bindings: stm32: do not disable spi-slave property for stm32f4-f7

STM32F4-F7 are, from hardware point of view, capable to handle device mode.
So this property should not be forced at false in dt-bindings.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/Message-Id

: <20230621115523.923176-3-valentin.caron@foss.st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>