soc: silabs: siwx91x: Add configurable power profile support via DeviceTree

This commit adds support for configuring the power/performance
profile for the siwx91x device using a generic
'power-profile' property in DeviceTree.
The property is available for NWP nodes,
allowing flexible selection of power management
profiles per application or board via overlay.

Signed-off-by: S Mohamed Fiaz <fiaz.mohamed@silabs.com>