openrisc: remove undefined of_get_cpu_node declaration

This patch removes the declaration of the function 'of_get_cpu_node'
which is not defined for openrisc. This is in preparation to move
it's definition from PPC to DT common code.

Again it could be there as it was originally copied from powerpc.

Acked-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>