ARM: dts: msm: Remove cpus-as-clocks property

CPUs have all moved to standard clock API, and this property is no
longer need for distinguishing CPU clock implementation. Remove
the property from DT and documentation.

Change-Id: I5dfff4e266e47546b1cb15f24993b870cc62fe67
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>