ARM: dts: msm: Modify qseecom for SDW2500

Modify qseecom memory reg for SDW2500 platform
external_image_mem is defined from 0x87b00000.
The oringe address of qseecom is started at 0x87a00000.
This address is not reserved for TZ apps.
This cause qseecom to access invalid address from 0x87a0000.

Change-Id: I24d80de3c84dc7812793cbc750ce6ec7bb8808cf
Signed-off-by: Baochu Xu <bxu@codeaurora.org>