add pwm freq adjustment

Author: ficeto

hardware/esp8266com/esp8266/cores/esp8266/Arduino.h

@@ -172,6 +172,7 @@ int digitalRead(uint8_t);
 int analogRead(uint8_t);
 void analogReference(uint8_t mode);
 void analogWrite(uint8_t, int);
+void analogWriteFreq(uint32_t freq);
 
 unsigned long millis(void);
 unsigned long micros(void);

hardware/esp8266com/esp8266/cores/esp8266/core_esp8266_wiring_pwm.c

@@ -139,4 +139,10 @@ extern void __analogWrite(uint8_t pin, int value) {
 	}
 }
 
+extern void __analogWriteFreq(uint32_t freq){
+  pwm_freq = freq;
+  prep_pwm_steps();
+}
+
 extern void analogWrite(uint8_t pin, int val) __attribute__ ((weak, alias("__analogWrite")));
+extern void analogWriteFreq(uint32_t freq) __attribute__ ((weak, alias("__analogWriteFreq")));