ext: simplelink: Restructure CC3220SDK as SimpleLink SDK

The SimpleLink SDK actually encapsulates a family of devices,
of which cc32xx is just one device.  Other devices can fit
under this SDK directory structure.

This expansion will also allow the import of the WiFi
host driver and its driver porting layer in the future,
inserted at the correct levels in the SimpleLink SDK
directory heirarchy.

Follow the URL (ending in "#directory-structure") referenced in
ext/hal/ti/simplelink/README which explains the SDK structure,
and where devices fit in.

Jira: ZEP-1958

Change-Id: I16515d3e3779de0d55d3b1b8e25029609d1f66c3
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>