msm: secure_buffer: Add SPSS_SP_SHARED vmid

Add support for SPSS_SP_SHARED vmid. This vmid is equivalent
to TZ VM, which can be used for sharing memory with TZ with
the assign call.

Change-Id: I10b98820796e8bab7b8275f5ddf44e8814dc0fb0
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>