subsys-pil_tz: Use a separate device for allocating metadata

Allocate metadata memory with a separate device so as
to not impose requirements on the memory associated with
the PIL device.

Change-Id: I4c817b7fdb1fe4f97d000481ab34c0fdf25e2781
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>