Hi,
You are reffering to 'IMR' registers in your post - but IMR are masks, not interrupt status registers.
Also I am not sure if you perform init in correct order.
You sholuld perform most of the USB initialization with USB cable unplugged then plug-in the USB cable and wait for the USB interrupt. This works for me.
And the best way to start is to ask Freescale support for their USB sample project for 5485.
Greetings,
Piotr