powerpc/85xx: consolidate of_platform_bus_probe calls

85xx board files have a lot of duplication in *_publish_devices()/
*_declare_of_platform_devices() functions. Merge that into a single
function common to most of the boards.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>