soc: silabs: siwx91x: Change memory partition

The current configuration allocate 476kB (672 − 196) to the NWP. This
configuration is only required with offloaded network stack
(CONFIG_WIFI_SILABS_SIWX91X_NET_STACK_OFFLOAD).

Since this parameter is not set by default, increase memory allocated to
Zephyr.

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