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...

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

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

Speed 24 MHz - not 24 Hz for Lowest voltage

CBT register for CANFD

Edited here on the web - copied to local machine and it built as tested : https://forum.pjrc.com/threads/54711-Teensy-4-0-First-Beta-Test?p=209959&viewfull=1#post209959

Proposed add locations for  startup_early_hook(); and startup_late_hook(); just before two existing waiting while()'s.
With debug 'PRINT_DEBUG_STUFF' disabled the three T4B2's here hit the EARLY code about 1.2 ms and LATE at about 45.3 ms after the systick is started.

Where the current printf_debug_init() call is might be critical at some point adjust 'something' as you did enabling the Serial4 port for debug - before clocks and other things are started perhaps startup_reset_hook()