soc: silabs: siwx91x: Allow alternative memory partition

Chip siwx91x has 672kB of SRAM shared between the Cortex-M4 (Zephyr) and
the NWP (Network Processor). 3 memory configurations are possible for
the Cortex-M4:
  - 196kB
  - 256kB
  - 320kB

Less memory is allocated to Zephyr, more memory is allocated to NWP,
better are the WiFi and BLE performances.

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