clk: imx8mn: correct the usb1_ctrl parent to be usb_bus

[ Upstream commit 134d43bb1ff09a696996f16ed8b28d404b770c8a ]

Per latest imx8mn datasheet of CCM, the parent of usb1_ctrl_root_clk
should be usb_bus.

Signed-off-by: Li Jun <jun.li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>