Raise PWM frequency

Adjusting PWM frequencies to better match FDD8780 MOSFET spec.

- Hotend PWM frequency increased from 488.28 Hz to 1953.12 Hz
– Referencing the Forward Bias Safe Area from the hotend MOSFET's datasheet, this value should be safer.
- Hotbed PWM frequency increased from 7.26Hz to 30.48Hz
- Use software PWM for fans to reduce whine
– The higher PWM frequency works well with the stock parts cooling fan and might allow for better control of third party fans

Marlin/Configuration.h

@@ -379,10 +379,10 @@
 
   // If you are using a pre-configured hotend then you can use one of the value sets by uncommenting it
 
-  // i3 Mega
-  #define  DEFAULT_Kp 15.94
-  #define  DEFAULT_Ki 1.17
-  #define  DEFAULT_Kd 54.19
+  // i3 Mega stock v5 hotend, 40W heater cartridge (3.6Ω @ 22°C)
+  #define  DEFAULT_Kp 18.58
+  #define  DEFAULT_Ki 1.38
+  #define  DEFAULT_Kd 62.40
 
   // Ultimaker
   //#define  DEFAULT_Kp 22.2
@@ -436,10 +436,10 @@
 
   //#define PID_BED_DEBUG // Sends debug data to the serial port.
 
-  //Anycubic i3 Mega 160W Ultrabase
-  #define DEFAULT_bedKp 251.78
-  #define DEFAULT_bedKi 49.57
-  #define DEFAULT_bedKd 319.73
+  //Anycubic i3 Mega Ultrabase (0.9Ω @ 22°C)
+  #define DEFAULT_bedKp 187.45
+  #define DEFAULT_bedKi 35.75
+  #define DEFAULT_bedKd 245.71
 
   //120V 250W silicone heater into 4mm borosilicate (MendelMax 1.5+)
   //from pidautotune
@@ -1827,13 +1827,13 @@
 // Use software PWM to drive the fan, as for the heaters. This uses a very low frequency
 // which is not as annoying as with the hardware PWM. On the other hand, if this frequency
 // is too low, you should also increment SOFT_PWM_SCALE.
-//#define FAN_SOFT_PWM
+#define FAN_SOFT_PWM
 
 // Incrementing this by 1 will double the software PWM frequency,
 // affecting heaters, and the fan if FAN_SOFT_PWM is enabled.
 // However, control resolution will be halved for each increment;
 // at zero value, there are 128 effective control positions.
-#define SOFT_PWM_SCALE 0
+#define SOFT_PWM_SCALE 2
 
 // If SOFT_PWM_SCALE is set to a value higher than 0, dithering can
 // be used to mitigate the associated resolution loss. If enabled,