ESP.getCpuFreqMHz fix (espressif#3007)

* ESP.getCpuFreqMHz was returning the CONFIG_ variable.  Now calls the getCpuFrequencyMhz function.

* Changed the Esp function to uint32_t to match

Author: lbernstone

cores/esp32/Esp.h

@@ -75,7 +75,7 @@ class EspClass
     uint32_t getMaxAllocPsram();
 
     uint8_t getChipRevision();
-    uint8_t getCpuFreqMHz(){ return CONFIG_ESP32_DEFAULT_CPU_FREQ_MHZ; }
+    uint32_t getCpuFreqMHz(){ return getCpuFrequencyMhz(); }
     uint32_t getCycleCount();
     const char * getSdkVersion();