Update DMAChannel.h

https://forum.pjrc.com/threads/63230-map()-returns-different-values-on-arduino-vs-teensy

Allow specific interrupt level for DMA ISR.

Fix size of arm_dcache_flush_delte on T4.x USB RAWHID

Was using SEREMU_TX_SIZE should use RAWHID_TX_SIZE

usb.c: ISR(USB_COM_vect): unused variable removed

https://forum.pjrc.com/threads/61410

Add digitalToggleFast for T4.x...

Like what was added for T3.x with that 3 yeard PR, I added the same support to T4.x

Added the PORT TOGGLE defines.  I copied the _PORTSET blocks and then simply used block editing in Sublime text to then change the SET to Toggle.

Also added in the code...

Test sketch:
```
void setup() {
  pinMode(13, OUTPUT);
}

int led = 13;
void loop() {
  digitalToggleFast(led);
  delay(500);
  digitalToggleFast(13);
  delay(250);
  digitalToggle(13);
  delay(1000);
  digitalToggle(led);
  delay(250);
}
```
Also added values to keywords.txt

Adds support for digital pin toggling digitalToggle(uint8_t pin)

https://forum.pjrc.com/threads/61412

Fix/Add seralEventUSB1 and serialEventUSB2

T4-HardwareSerial - RX look at FIFO

The code for T3.x was broken, especially for USB2...

And the code was not setup for T4.x.  So added it.

T4.x Reduce EventResponder overhead on systic and yield

I forgot to remove the default eventhandlers from some of the HardwareSerial files as such the ones in the serialEventX files was not used and why my flags were not set properly.

Could have done simple fix, but decide to make it more consistent with what I did in T3.x/LC code and not look through all the NULLS, but simply have list of active ones and count of active.

Appears to fix the issue so far.

Code is setup that if there are no eventResponders that have registered to do interrupt calls, don't have the systic code in place to look for it.

Also yield code.  Now figures out if the user has any code that installs their own serialEventX methods, to call them, but if they have left the default weak pointer versions, to not call them in yield.  Likewise will only look for eventRespnder events if the sketch has actually attached any events.  If none of these conditions are true, yield will just check one flag and return.

T3.x Serial events cleanup