ARM: dts: msm: Disable core_ctl for big cluster on SDM670

There is no plan to use core_ctl for big cluster (CPU#6-CPU#7). So
disable it. This avoids the additional overhead in the scheduler
hot paths.

Change-Id: Ic73963f15aa7acd427ea4d1bed24eac3662b2bd0
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>