fixes #524

Update usb_desc.h

Allow cores\teensy4 printf debug optionally go to USB

T3/T4.x MTP device descriptor - update endpoint and size

T4 compile fix - imxrt.h move )

Add defines for SNVS_LPGPR0-3 and GPR_Z_DIS

There was questions up on forum threads:

https://forum.pjrc.com/threads/66076-Teensy-4-x-s-1062-MCU-has-16-bytes-of-NVRAM-on-RTC-unit?p=268385#post268385

Originally in the thread: https://forum.pjrc.com/threads/64592-Teensyduino-1-54-Beta-5?p=268336&viewfull=1#post268336

That said that the 16 byte memory that was battery backup was not defined nor worked.

So added the defines for the 4 registers.  Only one early on alias was defined before this for the first one...

Also added in the SNVS_LPCR_GPR_Z define which the register bit was talked about in a few other register definitions, this bit was not actually defined in the reference manual, it said it was reserved.  But reads or writes without this bit set in the register will zero out the 4 registers.

Tried example sketch to make sure it worked

For T4.x to allow MTP USB type device to generate events to send to the host, the Endpoint configuration need to change to say that we are transmitting on interrupt, not receiving.

Plus I updated the end point event size to 32 as some events might be > 16.  16 is only large enought so send one parameter.  I noticed Kindle Fire has it set to something like 28...

I have done similar changes for T3.x but have not tested it yet.

allow multiple parameters for events

docs: fix simple typo, capbilities -> capabilities

There is a small typo in teensy3/usb_desc.c, teensy4/usb_desc.c.

Should read `capabilities` rather than `capbilities`.

add missing "WFI"

...prevent overheating when overclocking and exiting loop()

There are times when I wish to debug the cores files, that I wish I could do debug output to USB.  Note: can do with cpp files by doing Serial.printf or the like, but more of a pain with .c files.

The printf stuff put into cores has been very useful for debuging esperailly earlier on.  I understand some of the time it is needed to debug usb stuff or early on stuff so TX pin of Serial4 makes sense.

But for other debugging sessions, wish you had option to go output to the logical Serial object.

So I added a new commented out define in the printf.h file that syas
#define PRINT_DEBUG_USING_USB

That is uncommented, the debug function putchar_debug is defined to either use usb_serial_putchar if we are using normal USB Serial or use usb_seremu_putchar if using Serial emulation.

So far my main testing has been with seremu..

Not sure if this is something you are interested in.  but just in case.
Did this Pull Request

Update mk20dx128.c re FSEC use on 3.5/3.6

RE forum post : https://forum.pjrc.com/threads/65070-Teensy-3-6-and-FSEC-security-register?p=262560&viewfull=1#post262560

Same FSEC change works on T_3.5 as well as the T_3.6

Fix for https://github.com/PaulStoffregen/cores/issues/499

Update PXP defines for T4

Update usb.c

configure usb_mtp also from usb.c

Create usb_mtp.c

Create usb_mtp.h

modified and adapted from rawhid

mtp file derived from rawhid and modified slightly

Update HardwareSerial.cpp

If called second time - fix the counts to reflect just the size of the new buffer plus the static buffer.

Also as a precaution, resets the TX or RX head tail pointers back to 0 to make sure it is not indexing into memory that may not exist.

T3 x serial x add memory

T3.x,T4.x LC Serial half duplex support

Configurable Midi Port Names

T4 No USB Support

Update usb.c