ARM: dts: msm: add third cluster support for SDM855

SDM855 supports three clusters, update cpu map to reflect the same.

Change-Id: Id8f98d3ddf7f7ec1a0fcdc19052436d61d8c1941
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>