pmdomain: imx: Fix reference count leak in imx_gpc_probe()

pmdomain: imx: Fix reference count leak in imx_gpc_probe()

of_get_child_by_name() returns a node pointer with refcount incremented.
Use the __free() attribute to manage the pgc_node reference, ensuring
automatic of_node_put() cleanup when pgc_node goes out of scope.

This eliminates the need for explicit error handling paths and avoids
reference count leaks.

Fixes: 721cabf6c660 ("soc: imx: move PGC handling to a new GPC driver")
Cc: stable@vger.kernel.org
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>