SPI: provide a hook to set the minimum SPI frequency

On Giga, the system will complain loudly if the frequency selected is too low
```<err> spi_ll_stm32: Unsupported frequency 400000Hz, max 120000000Hz, min 937500Hz```
Let's try to keep this at minimum or change when we'll be able to grab the same infor from a macro/API provided by zephyr

Author: facchinm

Diff for: libraries/SPI/SPI.cpp

@@ -79,9 +79,13 @@ void arduino::ZephyrSPI::usingInterrupt(int interruptNumber) {
 void arduino::ZephyrSPI::notUsingInterrupt(int interruptNumber) {
 }
 
+#ifndef SPI_MIN_CLOCK_FEQUENCY
+#define SPI_MIN_CLOCK_FEQUENCY 1000000
+#endif
+
 void arduino::ZephyrSPI::beginTransaction(SPISettings settings) {
   memset(&config, 0, sizeof(config));
-  config.frequency = settings.getClockFreq();
+  config.frequency = settings.getClockFreq() > SPI_MIN_CLOCK_FEQUENCY ? settings.getClockFreq() : SPI_MIN_CLOCK_FEQUENCY;
   auto mode = SPI_MODE_CPOL | SPI_MODE_CPHA;
   switch (settings.getDataMode()) {
     case SPI_MODE0: