Fix clockCyclesPerMicrosecond Change from fixed value to current value (espressif#3993)

Author: tanakamasayuki

Diff for: cores/esp32/Arduino.h

@@ -78,7 +78,7 @@
 #define interrupts() sei()
 #define noInterrupts() cli()
 
-#define clockCyclesPerMicrosecond() ( F_CPU / 1000000L )
+#define clockCyclesPerMicrosecond() ( (long int)getCpuFrequencyMhz() )
 #define clockCyclesToMicroseconds(a) ( (a) / clockCyclesPerMicrosecond() )
 #define microsecondsToClockCycles(a) ( (a) * clockCyclesPerMicrosecond() )