boards: silabs: siwx91x: Add support for Flash

A specific section of the flash on the SiWx917 is reserved for the
Network co-processor (NWP). This zone needs to be delimited in order to
not overwrite it. We then need to use "code-partition" to provide Zephyr
the application code flash location.

Co-authored-by: Martin Hoff <martin.hoff@silabs.com>
Signed-off-by: Martin Hoff <martin.hoff@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>