This is a series of documents that intend to help you grasp he concept of the System Controller Unit (SCU) / System Controller Firmware (SCFW).
Other Useful resources:
You can get the latest System Controller Firmware Porting kit from the i.MX Software and development webpage:
The porting kit contains documentation and allows you to build the SCFW from source and do modifications to the board dependent parts:
The API documents explain the System Controller Firmware API syntax and it has a section for introduction and general overview of the system.
The port document contains information on what each board component function does and the overall process of porting the SCFW.