of/device: add helper to get cpu device node from logical cpu index

Multiple drivers need to get the cpu device node from the cpu logical
index and then access the of_node.

This patch adds helper function to fetch the device node directly.

Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>