MIPS: OCTEON: octeon-usb: add all register offsets

Glue code uses a mix of offset and absolute address register
definition. Define all of them as offsets and use them
consistently.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>