spi: stm32: add description about STM32F4 bindings

Add description that STM32F4 can be used in compatible property.
Master Inter-Data Idleness optional property cannot be used in STM32F4.

Signed-off-by: Cezary Gapinski <cezary.gapinski@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
1 file changed