cpufreq: qcom-hw: Add ready callback to register CPU cooling device

Add a ready callback, which will be called when a CPU policy is ready.
As a part of this callback, register the corresponding CPU cooling
device.

Change-Id: Ic12e0a8f7cd13fda583844454e3a0eb600a2cad3
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>