Update MIMXRT clocking to hopefully fix deep sleep, fix boot issues on Teensy4

[multiplemonomials]

Summary of changes

This MR was made in collaboration with testers on the Teensy forums, and fixes a number of issues:

• Incorrect boot header (__USED_FLASH_END instead of __USED_FLASH_SIZE) prevented boot
• Incorrect core clock on boot (see explanation on targets: clock: change default core clock to 528M ARMmbed/mbed-os#13391)
  • Added new "target.enable-overdrive-mode" option to control this
• Incorrect clocking configurations used when coming out of deep sleep (see targets: clock: change default core clock to 528M ARMmbed/mbed-os#13391)
  • To fix this, I pulled in updated lpm and clocking code from the latest SDK
  • This is a potential fix for MIMXRT106x: LPTicker does not work #115 , needs confirmation
• Incorrect pin mappings for UART3 and UART6 (daisy register settings were missing, I added them)
• .ncache section of memory was getting put into SEMC external RAM even if that ram was not enabled, causing a crash on boot. I have no idea why this got its own MEMORY section in the first place, and now I combined it into the main "m_data" section.
• Missing UART IRQ handlers for UARTs 5-8, causing a crash whenever those UARTs were used

Impact of changes

The TEENSY_40 target is now usable at a basic level! Unfortunately USB serial is still not working however, and it's missing a boatload of pin mappings.

Migration actions required

For those using MIMXRT10xxC chips, they need to specify "target.enable-overdrive-mode": 0 in their mbed_app.json to keep the core clock at 528MHz.

Documentation

---

Pull request type

[] Patch update (Bug fix / Target update / Docs update / Test update / Refactor)
[] Feature update (New feature / Functionality change / New API)
[X] Major update (Breaking change E.g. Return code change / API behaviour change)

---

Test results

[] No Tests required for this change (E.g docs only update)
[] Covered by existing mbed-os tests (Greentea or Unittest)
[X] Tests / results supplied as part of this PR

Testers on the pjrc forums have verified stuff for me.

---

Reviewers

---

[PaulStoffregen]

Forum conversation link:
https://forum.pjrc.com/threads/72107-Testers-Needed-Mbed-OS-Port-for-Teensy-4-0