MIPS: OCTEON: octeon-usb: move gpio config to separate function

Power gpio configuration is using Octeon specific code, so
move it to separate function, that can later be guarded
with ifdefs.

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