cpufreq: qcom-hw: Register child devices

Register child devices once a successful probe is done. This
will enable the dependent child devices to be probed immediately after
the cpufreq probe is done.

Change-Id: I9108636e1e6ad03bb6f5add9da9ba5507248c129
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>