msm: secure_buffer: Add helper function to convert VMIDs into strings

It can be useful to convert secure VMID values into strings.  Provide a
helper function to do so.

Change-Id: If907a0bac92c5d164154c0e5dfe67933115163c8
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>