ARM: dts: msm: Reduce secure image carvedout to 4MB for msm8909

Secure image size has been optimized and now needs
only 4MB carvedout instead of 7MB as previously
required. Reduce the size and correspondingly
change starting address of external image region.

Change-Id: Icdc7015eddbe11d68738d536df1f7a5445a045be
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>