Adds HardwareSerial Example (#8095)

New Example for using/testing Serial with any CPU/APB Frequency

libraries/ESP32/examples/Serial/Serial_All_CPU_Freqs.ino

+/*
+   Simple Sketch for testing HardwareSerial with different CPU Frequencies
+   Changing the CPU Frequency may affect peripherals and Wireless functionality
+   In ESP32 Arduino, UART shall work correctly in order to let the user see DGB info
+   and other application messages.
+
+   CPU Frequency is usually lowered in sleep modes
+   and some other Low Power configurations
+
+*/
+
+int cpufreqs[6] = {240, 160, 80, 40, 20, 10};
+#define NUM_CPU_FREQS (sizeof(cpufreqs) / sizeof(int))
+
+void setup() {
+
+  Serial.begin(115200);
+  delay(1000);
+  Serial.println("\n Starting...\n");
+  Serial.flush();
+
+  // initial information
+  uint32_t Freq = getCpuFrequencyMhz();
+  Serial.print("CPU Freq = ");
+  Serial.print(Freq);
+  Serial.println(" MHz");
+  Freq = getXtalFrequencyMhz();
+  Serial.print("XTAL Freq = ");
+  Serial.print(Freq);
+  Serial.println(" MHz");
+  Freq = getApbFrequency();
+  Serial.print("APB Freq = ");
+  Serial.print(Freq);
+  Serial.println(" Hz");
+  delay(500);
+
+  // ESP32-C3 and other RISC-V target may not support 240MHz
+#ifdef CONFIG_IDF_TARGET_ESP32C3
+  uint8_t firstFreq = 1;
+#else
+  uint8_t firstFreq = 0;
+#endif
+
+  // testing HardwareSerial for all possible CPU/APB Frequencies
+  for (uint8_t i = firstFreq; i < NUM_CPU_FREQS; i++) {
+    Serial.printf("\n------- Trying CPU Freq = %d ---------\n", cpufreqs[i]);
+    Serial.flush();  // wait to empty the UART FIFO before changing the CPU Freq.
+    setCpuFrequencyMhz(cpufreqs[i]);
+    Serial.updateBaudRate(115200);
+
+    Freq = getCpuFrequencyMhz();
+    Serial.print("CPU Freq = ");
+    Serial.print(Freq);
+    Serial.println(" MHz");
+    Freq = getXtalFrequencyMhz();
+    Serial.print("XTAL Freq = ");
+    Serial.print(Freq);
+    Serial.println(" MHz");
+    Freq = getApbFrequency();
+    Serial.print("APB Freq = ");
+    Serial.print(Freq);
+    Serial.println(" Hz");
+    if (i < NUM_CPU_FREQS - 1) {
+      Serial.println("Moving to the next frequency after a pause of 2 seconds.");
+      delay(2000);
+    }
+  }
+  Serial.println("\n-------------------\n");
+  Serial.println("End of testing...");
+  Serial.println("\n-------------------\n");
+}
+
+void loop() {
+  // Nothing here so far
+}