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-imx7ulp.c

@@ -57,7 +57,7 @@ static void __init imx7ulp_init_machine(void)
 
 	mxc_set_cpu_type(MXC_CPU_IMX7ULP);
 	imx7ulp_set_revision();
-	of_platform_default_populate(NULL, NULL, imx_soc_device_init());
+	of_platform_default_populate(NULL, NULL, NULL);
 }
 
 static const char *const imx7ulp_dt_compat[] __initconst = {