drivers: thermal: cpu_voltage: Use the child device node

The cooling device registration is using the parent node of the cpu
voltage cooling device. The devicetree configs are using the child node
to identify the cooling device. Switch to use the child device node, so
that the cooling device is bound with the thermal zone.

Change-Id: Id501421002c5dac7cb924455e09538a384ec9f85
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>