cpufreq: sun50i: replace of_node_put() with automatic cleanup handler

Make use of the __free() cleanup handler to automatically free nodes
when they get out of scope.

Signed-off-by: Javier Carrasco <javier.carrasco.cruz@gmail.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>