soc: imx9: don't access of_root directly

Don't access of_root directly as it reduces the build test coverage for
this driver with COMPILE_TEST=y and OF=n. Use existing helper functions
to retrieve the relevant information.

Suggested-by: Rob Herring <[e-mail]>
Reviewed-by: Peng Fan <[e-mail]>
Signed-off-by: Bartosz Golaszewski <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/soc/imx/soc-imx9.c

···
 	if (!attr)
 		return -ENOMEM;
 
-	err = of_property_read_string(of_root, "model", &attr->machine);
+	err = soc_attr_read_machine(attr);
 	if (err)
 		return dev_err_probe(dev, err, "%s: missing model property\n", __func__);
 
···
 	struct platform_device *pdev;
 
 	/* No match means it is not an i.MX 9 series SoC, do nothing. */
-	if (!of_match_node(imx9_soc_match, of_root))
+	if (!of_machine_device_match(imx9_soc_match))
 		return 0;
 
 	ret = platform_driver_register(&imx9_soc_driver);