powerpc/83xx: consolidate of_platform_bus_probe calls

83xx board files have a lot of duplication in
*_declare_of_platform_devices() functions. Merge that into a single
function common to most of the boards.

The only leftover is mpc834x_itx.c board file which explicitly asks for
fsl,pq2pro-localbus, as corresponding bindings don't provide
"simple-bus" compatibility in localbus node.

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