boards: cc3220sf_launchxl: Make cc3220sf XIP by default

Previously, there was no easy command-line solution for loading
a Zephyr program to (internal) flash.
So, the default development method was to load via gdb/openocd
to SRAM and debug from there, thus making the cc3220sf platform
non-XIP (CONFIG_XIP=n) by default.

With new openocd v 1.10 updates from TI (git.ti.com/sdo-emu),
the image can now be flashed and debugged via gdb/openocd,
so the default will be changed to XIP (CONFIG_XIP=y).

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