Merge branch 'feature/modify_pwm_ticks' into 'master'

Modify PWM TICKS3 to reduce CPU cost

See merge request sdk/ESP8266_RTOS_SDK!714

Author: donghengqaz

components/esp8266/driver/pwm.c

@@ -49,7 +49,7 @@ static const char *TAG = "pwm";
 #define AHEAD_TICKS0 0
 #define AHEAD_TICKS1 6
 #define AHEAD_TICKS2 8
-#define AHEAD_TICKS3 20
+#define AHEAD_TICKS3 2
 #define MAX_TICKS    10000000ul
 
 #define WDEVTSF0_TIME_LO     0x3ff21004