ext: simplelink: Import TI SimpleLink SDK host driver for CC3220SF

The SimpleLink SDK provides a WiFi host driver to access the
network coprocessor over a dedicated SPI.

This patch imports the host driver plus other driver files
required to communicate with the network coprocessor.

All files have been copied from a Linux SDK installation, without
modification (except for whitespace cleanup and file mode fixes).

Note that one of the imported files, wifi/porting/user.h, has to
be modified to enable the DPL (Driver Porting Layer) port to Zephyr.
This is provided in a follow-on DPL porting patch.

Origin: Texas Instruments SimpleLink CC3220 SDK URL:
http://www.ti.com/tool/download/SIMPLELINK-CC3220-SDK

 Purpose:
Provide WiFi host driver for TI CC32XX SoC

Jira: ZEP-1958

Signed-off-by: Gil Pitney <gil.pitney@linaro.org>