boards: silabs: siwx91x: Expose real layout of the flash

The Network Coprocessor on SiWx91x owns a large part of the flash. Zephyr
is not expected to access to theses areas.

However, it is still technically possible to access these. In addition, we
prefer the DTS contains a comprehensive and transparent description of the
hardware. So update the DTS with the real partitioning of the SoC.

Reference documentation is available here[1].

[1]: https://www.silabs.com/documents/public/application-notes/


     an1416-siwx917-soc-memory-map.pdf

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>