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

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()

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

To allow future MTP_SERIAL implementations and facilitate use of MTP_t4, that provides possibility of MTP_SERIAL

Fix dumb mistake on my part - thanksKurt

Update usb.c so MTP_repsonder can work at 600Mhz