Update DMAChannel.h

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

Allow specific interrupt level for DMA ISR.

Added in define in standard Arduino setup.

I also added for 7E1 and 7O1 as well.

Migrating the half duplex support I did in the Stagnant and out of date PR to enhance Uarts: https://github.com/PaulStoffregen/cores/pull/419

To try to support Half duplex mode in a similar way like we do for RS485 like support.

With T3.x made use of bitband address for the TX Direction setting, so
Except for the begin which calls format, no code changed, we simply stored the bitband address of the TXDIR flag into the TXDIR variable that was already used for the set direction flag.

For T4.x - it is a little more work as there is no bitband support on M7 processors.  For GPIO there is a Set and Clear register which we use, but for the appropriate UART register there is no such setup of registers.  So I have to special case we are in that mode and do it... Also since not atomic I cli/sei...

Serial Half Duplex - Fix T4Serial1 and T-LC Serial1-3

Updates: T4
T4 Serial 1 did not have proper settings for IOMUXC_LPUART6_TX_SELECT_INPUT

Tested T4.1 Serial1-8

Tested T3.5 Serial1-6

T3.6 - Added support for LPUART which is Serial6 which now works.

T-LC
Make half duplex work on T-LC on Serials1-3

Tested on on T3.5 1-6

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.