soc: qcom: pil: Reuse carveout region for mdt header

Currently we allocate a new dma region for our image headers to pass
into TZ for image verification. Instead reuse the previously allocated
region for the main firmware body to store the image header to avoid
having to allocate more memory than needed.

Change-Id: I1e50df2b417d9823c4e75f28134a3f9e078463aa
Signed-off-by: Kyle Yan <kyan@codeaurora.org>