ARM: dts: msm: Move plutonium /soc node within root node

Move the /soc node to improve performance of boot code that
needs to access the /cpus node without having to traverse
all MMIO devices and their properties.

Change-Id: I7d129a7b445b1add65eb362125f08c88ee04218b
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>