Zephyr: add nrf5340dk_nrf5340_cpuapp board

# TODO

Add the relevant overlay files to be able to build at least the mini sample on this board.

This board has very limited RAM and flash, but it should be able to run at least the mini sample.

## Required

- [ ] `mini` sample

## Optional but desirable:

- [ ] `demo` sample
- [ ] `supervisor` sample


# Out of scope:

- build in the CI
- real device testing