MIPS: OCTEON: octeon-usb: introduce dwc3_octeon_{read,write}q

Move all register access code into separate functions and
provide their no-op version for non Octeon platforms.

Later it might be possible to replace them with standard
Linux functions, however datasheets are not publicly available
and I have only one Octeon board to test, so lets stay on safe
side for now.

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