spi: stm32: deprecate `st,spi-midi-ns` property

The `st,spi-midi-ns` property, which was used to set a nanosecond delay
between transferred words, is now deprecated.
This functionality is now supported by the SPI framework through the
`spi_transfer` struct's `word_delay` variable.
Therefore, the private `st,spi-midi-ns` property is no longer needed and
has been deprecated in favor of the generic solution.

Signed-off-by: Clément Le Goffic <clement.legoffic@foss.st.com>
Link: https://patch.msgid.link/20250616-spi-upstream-v1-5-7e8593f3f75d@foss.st.com


Signed-off-by: Mark Brown <broonie@kernel.org>