soc: qcom: secure_buffer: Move to subsys_initcall

Move the secure buffer driver initialization to subsys_initcall
to avoid delaying the probe of drivers that depend on the
secure buffer driver when all of the drivers are builtin.

Change-Id: Ifdb1003c1c64df4371057528d1f1a0860e79c454
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>