ARM: imx: use device_initcall for imx_soc_device_init

This is preparation to move imx_soc_device_init to drivers/soc/imx/

There is no reason to must put dt devices under /sys/devices/soc0,
they could also be under /sys/devices/platform, so we could
pass NULL as parent when calling of_platform_default_populate.

Following soc-imx8.c soc-imx-scu.c using device_initcall, need
to change return type to int type for imx_soc_device_init.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/mach-imx/mach-imx6ul.c

@@ -55,13 +55,7 @@ static inline void imx6ul_enet_init(void)
 
 static void __init imx6ul_init_machine(void)
 {
-	struct device *parent;
-
-	parent = imx_soc_device_init();
-	if (parent == NULL)
-		pr_warn("failed to initialize soc device\n");
-
-	of_platform_default_populate(NULL, NULL, parent);
+	of_platform_default_populate(NULL, NULL, NULL);
 	imx6ul_enet_init();
 	imx_anatop_init();
 	imx6ul_pm_init();