clk: imx8mn: Add missing of_node_put()

After finishing using device node got from of_find_compatible_node(),
of_node_put() needs to be called.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>