PM / devfreq: rk3399_dmc: Add missing of_node_put()

of_node_put() needs to be called when the device node which is got
from of_parse_phandle has finished using.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>