cc3200: Added board documentation in RST format.

This follows the board.rst template in the RFC
for board documentation:
https://gerrit.zephyrproject.org/r/#/c/9703/

This RST file was validated visually by rendering
using the online tool: http://rst.ninjs.org/



JIRA: ZEP-1541

Change-Id: Ibe6e1c7f8eb3c7862b4087a78b469693927280a2
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>