CYW43xx WiFi SoC driver

This is a driver for the CYW43xx WiFi SoC.

There are four layers to the driver:

1. SDIO bus interface, provided by the host device/system.

2. Low-level CYW43xx interface, managing the bus, control messages, Ethernet frames and asynchronous events. Includes download of SoC firmware. The header file cyw43_ll.h defines the interface to this layer.

3. Mid-level CYW43xx control, to control and set WiFi parameters and manage events. See cyw43_ctrl.c.

4. TCP/IP bindings to lwIP. See cyw43_lwip.c.