soc: silabs: siwx91x: Initialize NWP sufficiently early

The NWP provides Bluetooth, Wi-Fi and crypto services, and must
be initialized before any users of these. Mbed TLS is initialized
at priority level 40 (kernel default priority), ensure that the
NWP is initialized before that.

Make the priority configurable to allow users to tweak the init
sequence.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>