ARM: dts: msm: update to memory map for sdx24

Increase modem size from 135MB to 139MB on 512MB variant as per v3
memory map and shift modem starting address down by 4MB on 256MB
variant as per v8 memory map to sync up with the starting address
of 512MB and 256MB variants for SDX24.

Change-Id: I037be43642671cca98581339814292fe3972036f
Signed-off-by: Vishwanath Raju K <vishk@codeaurora.org>