firmware: imx-scu: fix OF node leak in .probe()

imx_scu_probe() calls of_parse_phandle_with_args(), but does not
release the OF node reference obtained by it. Add a of_node_put() call
after done with the node.

Fixes: f25a066d ("firmware: imx-scu: Support one TX and one RX")
Signed-off-by: Joe Hattori <joe@pf.is.s.u-tokyo.ac.jp>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>