arm: omap3: Make secureworld_exit() static

secureworld_exit() is only used in one file, so make it static
to that file and remove it from sys_proto.h. This
may help with some further optimization in the future.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210625192308.277136-3-aford173@gmail.com