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>