arm: omap3: Make secure_unlock_mem() static

secure_unlock_mem() is only used in one file, so make it static
in that file.

Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210625192308.277136-4-aford173@gmail.com