libraries/SPI: Add support for peripheral mode.

[iabdalkader]

Requires:

• Add SPI bus mode support (controller/peripheral) ArduinoCore-API#257

Example:

#include <SPI.h>

SPISettings mySettings(1000000, MSBFIRST, SPI_MODE0, SPI_PERIPHERAL);

void setup() {
  Serial.begin(115200);
  while (!Serial);
  SPI.begin();
}

void loop() {
  uint8_t buf[] = {0x01, 0x02, 0x03, 0x04};

  SPI.beginTransaction(mySettings);
  SPI.transfer(buf, sizeof(buf));
  SPI.endTransaction();
  
  Serial.println("Received:");
  for (int i=0; i<sizeof(buf); i++) {
    Serial.print("0x"); Serial.println(buf[0], HEX);
  }
  
  delay(100);
}

[facchinm]

Let's just add SPI_HAS_PERIPHERAL_MODE define and we are good to go 🚀

[iabdalkader]

Let's just add SPI_HAS_PERIPHERAL_MODE define and we are good to go 🚀

Added.