spi: dt-bindings: st,stm32-spi: Move "st,spi-midi-ns" to spi-peripheral-props.yaml

spi: dt-bindings: st,stm32-spi: Move "st,spi-midi-ns" to spi-peripheral-props.yaml

In order to validate SPI peripherals, SPI controller-specific child node
properties need to be in a separate schema, spi-peripheral-props.yaml,
which SPI peripheral schemas reference. As there is just a single
property in this case, just add it to spi-peripheral-props.yaml directly.

Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230914190049.1853136-1-robh@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>