undocumented SNVS regs

use GPIO6..9

add SNVS definitions for RTC

add #define digitalPinToInterrupt(p)

todo:
analogInputToDigitalPin(p)
digitalPinHasPWM(p)

Add SD Cards as IO pins - plus fix Serial2 and Serial8 for T4 B2

Fixed the Serial2 object to use the new correct pins, plus made Serial8 only valid for T4 B1 (not B2).

Also I defined IO pins for the six signals associated with the SD Card slot.

I have tested these both as IO pins and now as SPI2

update setpad in Analog.c and added debug info in startup

Fix SPDF Base-Address

Startup: save RAM with .progmem

T4 Imxrt define Flexio structure.

Moved from my flexio_t4 library.
Libary was updated to see if already defined before it now defines it.

clock halved - halve blink wait count

more defs

For review - Expose alt weak func(), label displayed dump

pendablesrvreq_isr = priority 32;

add some definitions for audio

speed optimize intervalTimer

Code from :: https://community.nxp.com/thread/389002
Name the fault dump info
New weak to override :: HardFault_HandlerC()

T4 - setTX with open drain mode.

Allow the user to call setTX with the one hardware pin, and hopefully allow them to turn on Open Drain mode or turn it back off.  I don't currently have any test cases that use this, so not sure how well it works.

add nullptr check to Print.cpp - write