To allow future MTP_SERIAL implementations and facilitate use of MTP_t4, that provides possibility of MTP_SERIAL

Wondered  if this had already been done earlier.

Was not so am trying it now.

One side effect was had to change interrupt.c as there were
#defines for things like DR, GDIR which then caused compile issues with the structure.  So renamed the defines.

Built about 3 programs with it so far.  Which appear to still be working.

Missed a few, pllus one register fields name had wrong register name

https://forum.pjrc.com/threads/63842

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

Allow specific interrupt level for DMA ISR.

Was using SEREMU_TX_SIZE should use RAWHID_TX_SIZE

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

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