Added LEDC Software Fade example (#202)

* Added LEDC Fade exmaple

* Renamed LEDCFade example to LEDCSoftwareFade

Added valueMax parameter for ledcAnalogWrite

* Remove usage of LED_BUILTIN constant to solve CI errors

libraries/ESP32/examples/AnalogOut/LEDCSoftwareFade/LEDCSoftwareFade.ino

+/*
+ LEDC Software Fade
+
+ This example shows how to software fade LED
+ using the ledcWrite function.
+
+ Code adapted from original Arduino Fade example:
+ https://www.arduino.cc/en/Tutorial/Fade
+
+ This example code is in the public domain.
+ */
+
+// use first channel of 16 channels (started from zero)
+#define LEDC_CHANNEL_0     0
+
+// use 13 bit precission for LEDC timer
+#define LEDC_TIMER_13_BIT  13
+
+// use 5000 Hz as a LEDC base frequency
+#define LEDC_BASE_FREQ     5000
+
+// fade LED PIN (replace with LED_BUILTIN constant for built-in LED)
+#define LED_PIN            5
+
+int brightness = 0;    // how bright the LED is
+int fadeAmount = 5;    // how many points to fade the LED by
+
+// Arduino like analogWrite
+// value has to be between 0 and valueMax
+void ledcAnalogWrite(uint8_t channel, uint32_t value, uint32_t valueMax = 255) {
+  // calculate duty
+  uint32_t duty = (LEDC_BASE_FREQ / valueMax) * min(value, valueMax);
+
+  // write duty to LEDC
+  ledcWrite(channel, duty);
+}
+
+void setup() {
+  // Setup timer and attach timer to a led pin
+  ledcSetup(LEDC_CHANNEL_0, LEDC_BASE_FREQ, LEDC_TIMER_13_BIT);
+  ledcAttachPin(LED_PIN, LEDC_CHANNEL_0);
+}
+
+void loop() {
+  // set the brightness on LEDC channel 0
+  ledcAnalogWrite(LEDC_CHANNEL_0, brightness);
+
+  // change the brightness for next time through the loop:
+  brightness = brightness + fadeAmount;
+
+  // reverse the direction of the fading at the ends of the fade:
+  if (brightness <= 0 || brightness >= 255) {
+    fadeAmount = -fadeAmount;
+  }
+  // wait for 30 milliseconds to see the dimming effect
+  delay(30);
+}