boards: Add non-secure target for nrf54L15dk

This adds the nrf54l15dk/nrf54l15/cpuapp/ns board variant to
Zephyr. It allows to build applications for this target.

This is an initial support for the non secure target which allows
building and running tfm_ipc and config_build.

This is NOT full support of the non secure target in upstream
Zephyr.

There are important limitations, such as:
- The hardware Crypto accelerator is not supported and thus the non
  secur target is NOT secure for production applicatiions in upstream
  Zephyr.
- The BL2 is not supported, so no DFU is supported with this support

Most of the code chagnes here are taken from nRF Connect SDK
in order to avoid having noups there.

Signed-off-by: Georgios Vasilakis <georgios.vasilakis@nordicsemi.no>