firmware: imx: scu: change init level to subsys_initcall_sync

Change firmware init level to subsys_initcall_sync to ensure it's
probed before most devices to avoid unnecessary defer probe.

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