boards: silabs: siwx91x: Fix compatibility with tests

Some tests (eg. tests/bluetooth/shell) expects to have a
storage_partition defined.

storage_partition is also defined in the DT of most of the boards.

For now, this patch define a very small storage partition to avoid any
conflict with code_partition.

Tested-by: Andras Nyiri <andras.nyiri@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>