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>

Documentation/devicetree/bindings/spi/spi-stm32.txt

@@ -7,7 +7,9 @@ from 4 to 32-bit data size. Although it can be configured as master or slave,
 only master is supported by the driver.
 
 Required properties:
-- compatible: Must be "st,stm32h7-spi".
+- compatible: Should be one of:
+  "st,stm32h7-spi"
+  "st,stm32f4-spi"
 - reg: Offset and length of the device's register set.
 - interrupts: Must contain the interrupt id.
 - clocks: Must contain an entry for spiclk (which feeds the internal clock
@@ -30,8 +32,9 @@ Child nodes represent devices on the SPI bus
   See ../spi/spi-bus.txt
 
 Optional properties:
-- st,spi-midi-ns: (Master Inter-Data Idleness) minimum time delay in
-		  nanoseconds inserted between two consecutive data frames.
+- st,spi-midi-ns: Only for STM32H7, (Master Inter-Data Idleness) minimum time
+		  delay in nanoseconds inserted between two consecutive data
+		  frames.
 
 
 Example: