cc3200: Add a GPIO driver for the TI CC3200 LaunchXL

The pinmux configuration is done during board initialization.

This was validated using the following Zephyr apps:
 - samples/basic/blinky
 - samples/basic/disco
 - samples/basic/button

All 4 GPIO ports are supported.

Change-Id: If8599a23c1d56cfd678a6e2e5339f7e093c6061a
Signed-off-by: Gil Pitney <gil.pitney@linaro.org>