dts: msmplutonium: Add support for 8 cores

We add CPU nodes for the 4xCortex-A57s which enables us to
boot with 8 cores.
Also, use the correct compatible string for the CPU nodes.

Change-Id: Ib52c91926381085dc56f8a04df28029017e708c0
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>