MIPS: Octeon: Move some platform device registration to its own file.

There is a bunch of platform device registration in
arch/mips/cavium-octeon/setup.c.  We move it to its own file in
preparation for adding more platform devices.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>