soc: qcom: kconfig: Relax QCOM_MEM_BUF dependencies

The mem-buf driver doesn't strictly depend on the secure
buffer functionality. For example, it can still be functional
in environments where it is used purely for requesting memory
from other VMs. Thus, remove the dependency.

Change-Id: Ie4e39c00473509ee3552e6da849c633ccfab21ce
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>