of: return of_get_cpu_node from of_cpu_device_node_get if CPUs are not registered

Instead of the callsites choosing between of_cpu_device_node_get if the
CPUs are registered as of_node is populated by then and of_get_cpu_node
when the CPUs are not yet registered as CPU of_nodes are not yet stashed
thereby needing to parse the device tree, we can call of_get_cpu_node
in case the CPUs are not yet registered.

This will allow to use of_cpu_device_node_get anywhere hiding the
details from the caller.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>