The iMX RT has a separate wake up controller, the GPC, that replaces
the NVIC when asleep. It adds the ability to only wake up on certain
interrupts. It seems that it requires at least one enabled interrupt
in the NVIC to turn on it's wake up circuitry. It doesn't need to
be the same interrupt as the wake up signal. For example, the RTC
in the SNVS can wake us up if a USB interrupt is enabled. Before
then it won't work. So, we enable the SNVS interrupt on start up
so it can wake us up.
Introduces a way to place CircuitPython code and data into
tightly coupled memory (TCM) which is accessible by the CPU in a
single cycle. It also frees up room in the corresponding cache for
intermittent data. Loading from external flash is slow!
The data cache is also now enabled.
Adds support for the iMX RT 1021 chip. Adds three new boards:
* iMX RT 1020 EVK
* iMX RT 1060 EVK
* Teensy 4.0
Related to #2492, #2472 and #2477. Fixes#2475.
