ARM:dts:msm: update memory map for apq8009

Increase externel_image_mem size to 7MB from 6MB for
APQ8009 based apq8009-robot-som-refboard platform.

Change-Id: Icb421f4795e4f1d15e94d11944332613c6d10a0c
Signed-off-by: Vishwanath Raju K <vishk@codeaurora.org>