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

Added support for retrieving the regulatory domain information from
the siwx91x driver. Since the SDK does not provide a GET API for
region details, the driver now stores the country code and reuse
the configuration `sli_si91x_set_region_ap_request_t` to get the
channel information. This stored data is returned when a GET
operation is requested.

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