drivers: wifi: siwx91x: Add support for setting regulatory domain

Add support for configuring the regulatory domain (region code) to
ensure the device operates in compliance with local RF regulations.

The region must be set before the Wi-Fi interface becomes active
i.e., before scan or connection in STA mode, or before starting in
AP mode.

Since the SDK does not support setting the country code directly,
the driver maps country codes to region codes

Signed-off-by: Arunmani Alagarsamy <arunmani.a@silabs.com>