Add ADC Structure and redefine using, plus XBAR defines.

Update imxrt.h for encoder structure

Add structure to make it easer to use ADC objects,

Plus updated defines to use the structure.
Left in my own define for structure location as a trigger to not define it in my own code when running older version of core...

Also added defines for XBAR, such that we can hopefully not have to use magic numbers to say, map PID1 to ADC_ETC1_...

T4 avr registers

It was not properly clearing the old state for the ADC_CFG_AVGS, but instead it was oring in the new value,

also T4-Analog fix calibration tests
Found by @mjs513 - At init time it was not looking at ADC2 to have completed calibration...

Remove T41b1 pins ifdef

The TX and RX values were reversed.
That is for example on Serial1 (LPUART6)
Before we had:
```
```
now we have
```
```
So for example trying to get WS2812Serial library to work was failing, to output anything.

Had wrong pin in my table, so fixed table.

User found on forum: https://forum.pjrc.com/threads/58414-Teensy-4-0-Serial3-attachCts-wrong-pin-in-HardwareSerial3-cpp?p=221464#post221464

Here is a first pass at trying to get the AVR emulation of Ports working like it did for T3.x.

Not 100% confident, but at least simple sketch is working.

Reading wrong register for one bit...

Needed to implement Quad encoder library.

Even commits from master

Rename CCM_CCGR1_GPT defines

Current naming is confusing since it differs between GTP1 and GTP2. The GTP2 version is compatible to the naming in the data sheet. So I suggest to adjust the naming for  GPT1

As per the Teensyduino 1.48 release thread.

T3.x sketches could compile that had int mai() {} and not void() and setup():

So simply copied over setup.cpp from T3 folder of cores, and then changed setup.c to call main, which simply calls setup and loops calling loop/yield.

Added another endless loop after the call to main() to hang there if the users main returns.

Tested with simple sketch
```
#include <arduino.h>
int main (void) {
  pinMode(13, OUTPUT);
  for(;;) {
    digitalWriteFast(13, HIGH);
    delay(250);
    digitalWriteFast(13, LOW);
    delay(250);
  }
}
```
tried it on T3.6 verified before compile failed and now works.  Also compiled and run a normal app to just make sure it was not totally busted

Teensy4: Startup Hooks

fixes #394

Serial8 isn't a keyword now

https://forum.pjrc.com/threads/57360-Teensy4-IntervalTimer-Channels-Problem

Explicitly declare TMR channel registers in IMXRT_TMR_t

I suggest to explicitely delcare a (say)  IMXRT_TMR_CH_t struct which collects the registers for a TMR  channel instead of implicitely defining this struct in the declaration of IMXRT_TMR_t.

This would allow for an easier definition of user classes modelling a TMR channel.
Not a big deal, could also do that mayself the class code but it would be more convenient if this would be done in imxrt.h