drivers: wifi: Introduce SiWx91x WiFi driver

This driver allow to use Zephyr native IP stack or the IP stack provided
by HAL / WiseConnect.

The WiseConnect implementation may take advantage of the specific
features provided by the 917 (power consumption, speed,
validation...).

Some notable features are not available with this interface:
  - It seems Zephyr does not provide API to offload multicast membership
    management. User should be to directly call WiseConnect APIs
  - Support for ICMP frames is difficult. Note that WiseConnect
    automatically answer to ping request. It is just not possible to
    send ping requests and receive ping responses.
  - Zephyr and WiseConnect both support TLS offloading. However this
    patch does not implement it.
  - Reentrancy in the WiseConnect side is uncertain.

This implementation has been tested with samples/net/wifi/ (which relies
on subsys/net/lib/shell).

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